Perhaps she had insulted them.
Maybe there was a delicate social order she upended at some point she couldn't quite recall. Maybe it was just the violent tendencies hardwired into the brains of all foals between the ages of five and twelve coming out. The reason, Twilight considered, actually mattered very little. The fact remained that the moment Twilight entered the schoolyard, she found herself surrounded by somewhere between ten or fifteen schoolfoals, all with a murderous intent in their eyes. Twilight frowned, rather unsure of what to do with the situation presented to her. Sure she could simply brush them aside with her magic, but that would hardly deter them for very long. Foals could be quite persistent. However, she didn't want to hurt the little bastards too badly, just give them enough of a scare that they didn't try this again.
Twilight took a half step backwards when she saw Dinky and Featherweight take a step towards her, the latter menacingly cracking his neck. She considered her rather lacking arsenal of low-powered combat magic and grimaced. Nothing she thought of could take them out without causing enough damage that she'd be sued for foal abuse again.
She might have to use her hooves for this one. Good. More of a challenge that way.
Suddenly, her princess-sense went wild, and she felt that some fiendish rogue decided that sneaking up on her right be a good idea. Said rogue failed to consider it tended to be a bad idea to sneak up behind horses or ponies or... whatever, because they'll do exactly what Twilight did: she put all of her weight on her forehooves and bucked out with at least four elevenths of her power. She heard a grunt of pain and the whistling noise of a foal flying through the air, and she chanced a glance back to see the fat one with the stupid fez whose name she could never remember landing in the dirt.
The battle had officially begun.
The remaining foals all charged at once, forcing Twilight to go on the defensive, or rather, a strategic retreat with her wings to wherever the foals weren't. She spread her wings and hopped over the mass of charging young 'uns, spinning in the air a little to get a good grasp of her situation. Her maneuver did some good, as one or two of the foals had been going quickly enough that they could not stop themselves, and as such, they ran into each other, knocking themselves to the dirt. Twilight grinned and landed behind the tiny mass. Using the fact that they didn't know where she was to her advantage, Twilight grabbed Snips, stood on her hind legs, and hurled him as hard as she could.
While she could never publicly admit it, given that her status in life meant that some compassion was always necessary, the bowling pin sound of Snips colliding with the rest of them gave Twilight no shortage of internal glee. No time to dwell on that more, though. They weren't out for the count yet. Five more foals leaped up and charged her, two of them taking to the air to ensure that Twilight couldn't escape them that way. Twilight narrowed her eyes and charged right back at them, not missing the fact that the flying foals were right at eye level with her. They seemed to be planning to gouge her eyes out, if the position of their extended hooves was any indication. However, at the last possible second, Twilight ducked low and shot one hoof up as the foals passed harmlessly overhead. She felt her hoof collide with the soft, squishy underbelly of one of her attackers, knocking her out for the count. With their air defenses partially neutralized, Twilight was able to effortlessly hop over the ground-bound foals charging her.
Twilight took to the air once more, scanning the foals circling below her like sharks, to assess her situation. Snips, that fez pony, and the filly she had punched in the gut seemed out for the count, but that left eleven more to deal with, none of whom seemed particularly nervous that they hadn't landed a blow, yet already lost three of their fellows to her remarkable fighting prowess. However, three of them seemed to have some sense and were trotting over to the swing set instead. Yes, better to play on those then feel the wrath of an alicorn, however subdued that wrath may be.
With only eight more, Twilight flipped around and charged downwards towards her targets, one of her forelegs extended in front of her. Her target, Snails, stood his ground, not even blinking as he faced his upcoming lavender, alicorny serving of pain. Twilight grinned and licked her lips, but her smugness was cut off when she felt something collide with her side. She yelped in shock more than pain when she skidded across the dirt ground. She tried to reorient herself when she came to a stop, but flinched back when she felt a tiny hoof connect with her head just under her right eye. She hissed in pain and rolled on the ground just in time to avoid a second blow from what turned out to be Scootaloo. The filly in question, seeing that her target had escaped, whirled around and leaped at Twilight once more with a shout of "for Rainbow Dash!"
Twilight didn't even have time to consider how misguided this statement was, considering the fact that Rainbow Dash was far closer to Twilight than this orange nuisance, as when she side-stepped the attack, she happened to do so right into the maliciously eager hooves of the charging crowd of foals. With a surprising amount of strength, and a little magic from the unicorns, they piled on top of her and began their assault in earnest.
She was kicked, she was scratched, she was given wet willies and, most importantly, a lock of her mane was cut off with some sort of cutting spell from Sweetie Belle. An important lock, no doubt. Twilight struggled, but the youthful mass proved to be quite heavy, and the frequency of the foals' blows proved distracting enough that she couldn't escape, particularly when they punched her horn.
With a growl, Twilight decided the time might be now to fight a little dirty. She opened one eye, the one not already blackened from Scootaloo's punch, and saw Featherweight and Snails in front of her. She reached out, grasped both of their heads, and slammed them together. The two fell without a sound, completely knocked out. Twilight grinned. An opening at last! She quickly scooted forward and took to the air when she was clear enough of the pile.
As she stared down at the thankfully non-flying ponies below her, the thought occurred to her that maybe she should abandon all of this. Not only was she fighting foals, but they were putting up a rather good fight too. Spike would be most displeased when she came home looking like she did. When was she going to create that royal guard she had drawn all of those wonderful, masculine, steamy sketches of? Surely the ones already down would realize the error of their ways. She could quit for today.
Wait...
No...
That wasn't how she acted. She was a princess of Equestria, herself dammit, and if she just gave up while fighting a schoolyard full of foals, no foreign dignitary could ever take her seriously again! Worse still, she'd never hear the end of it at poker nights with the other three princesses. No she needed to beat the cutie marks off of, or onto, these foals to prove that she was a true princess! The Cutie Mark Crusaders, Diamond Tiara, Silver Spoon, and Dinky. That's all she had left. A piece of cake.
Before she could think of the best way to take the six of them out, she felt something wrap around her right foreleg, then her left. She blinked once, then saw that the three schoolfoals who had left for the swings were back. It seems that they weren't retreating, but merely gathering improvised means of keeping her grounded in case she took to the air again, as they had removed the swings from where they hung, created lassos with the rope ends, roped her forelegs, and were now in the process of pulling her back down to the ground. Twilight began struggling to take to the air again, but the combined weight of all nine foals meant that even with her earth pony strength, she was weighted down. The ropes were too tight on her forelegs, so that was going to be a problem, even if she did manage to take flight again.
It looked like ground combat time again.
Using her earth pony strength, Twilight whipped the left side of her body around and flung the four foals holding that rope aside. Only Dinky managed to hold on, with the other three foals who had gone to get the swings being tossed into the fence. She then did the same for her other side, but the other five were ready for her, and let go of their rope just as she swung. With nothing to channel her momentum into she flung herself too far and fell to the ground.
Seeing their prey immobilized for the moment, the foals charged at Twilight, but she wasn't out of it yet. She rolled out of the way as well as she could with her forelegs still tied, and managed to gain enough ground to push herself to a standing position. She raised her right foreleg and began whirling the swing around, snapping out with it as the foals approached. The wood of the swing smashed into Diamond Tiara's head, knocking her out cold.
Now the foals found themselves faced with a conundrum. Instead of immobilizing her like planned, Twilight now had a weapon, but things had gone too far now. There was honor to be gained and lost on both sides should either one call it off now. The fight needed to be seen to the end. They all knew that. Twilight continued twirling the swing over her head, ready for the next foal foolish enough to charge at her. That foal turned out to be Silver Spoon, who tried a little feint before charging directly, but Twilight was ready. When Silver Spoon came too close, Twilight brought the board of the swing down on her head, easily dispatching her. However, she failed to consider that Silver Spoon's assault might be a ruse. She learned quickly. With her other hoof on the ground, the other swing lay immobile as well. Apple Bloom saw her chance and, with her own earth pony strength, jumped up on the board, cracking it in half. Twilight yelped at the destruction of her weapon and pulled the ruined swing away, but Apple Bloom planned for that as well, and she jumped on the other swing too, breaking it as well.
Now that the swings were broken, what was once her weapon now became a hindrance once more. Her remaining four attackers stood before her, slowly advancing. With her hooves tied as they were, a single slip-up could result in her on the bottom of a foal pile being attacked again, and she preferred that not happen.
Dinky and Sweetie Belle exchanged a wicked grin that Twilight didn't like, and both of their horns fired up at the same time. The ends of the ropes began tying themselves together with surprising speed, and Twilight realized that she might need to use some magic to get out of this one, thereby dulling the thrill of using her hooves.
That and she resolved to never teach Sweetie Belle anything ever again.
With a war cry that only princesses knew how to replicate, Twilight leaped forward with her wings, her forelegs outstretched, and clotheslined Sweetie Belle and Dinky. She grabbed them in her forelegs and slammed their heads together before lifting Sweetie Belle into the air and swinging her at Scootaloo like a flail. Given that she possessed no fingers, the blow made her lose her grip on her marshmallowy weapon, but it didn't matter. She landed and spun around to face a glaring Apple Bloom. Her final opponent.
The two stared down at each other, Twilight breathing heavily but Apple Bloom barely winded. Apple Bloom cracked her neck and removed her hair bow from her mane, letting her mane fall to her shoulders, before tying it around her right foreleg.
"So, Twilight." Apple Bloom took a step forward and pawed at the ground. "It's down ta just you 'n me now. Ah suppose we'd better just do this the honorable way mah sister taught me an—"
Apple Bloom's words were cut off when Twilight charged forward and slammed her hoof into Apple Bloom's head sort of almost as hard as she could, but not really because Apple Bloom was a filly. She fell to the ground completely unconscious.
Twilight blinked once and looked around the schoolyard. The few foals not unconscious groaned and tried dragging themselves towards the schoolhouse. Twilight grinned in victory and used her magic to untie her forelegs.
"It just goes to show you: You can't beat a princess at this kind of—"
Twilight turned to the schoolhouse, and her words died in her throat when she saw who stood there. Cheerilee stood on the steps of the school, her eyes wide and her jaw hanging almost to the ground. Twilight blushed and flattened her ears before slowly walking up to the schoolteacher. When Cheerilee saw her coming, she closed her eyes and mouth and gave Twilight a small glare.
"Er... hey, Cheerilee." Twilight bit the inside of her lip. "How long have you been standing there?"
Cheerilee took a slow, deliberate breath and stared out into the battlefield of battered and bruised foals. "Long enough, Twilight."
"Yeah." Twilight motioned back to the mess on the playground. "Sorry about this but we had... well, I don't really know what happened. It all kind of went by in a rush, you know?" Cheerilee failed to reply, so Twilight chuckled nervously and scratched the back of her neck.
"So... I take it that you don't want me guest lecturing on the history of Winter Wrap-up anymore?"
In the background one of the foals groaned and muttered something about the pain. Cheerilee let out an exasperated grunt and slowly brought her hoof up to her face. "N... no, Twilight. I do not want you to do a guest lecture on the history of Winter Wrap-up anymore."
Twilight shrugged and gave Cheerilee a cheerful smile completely inappropriate for the situation. "Well, okay, then. Let me know if you change your mind."
With that, Twilight spread her wings and began flying back to the library, leaving Cheerilee staring blankly at the scene before her.