Twilight has been having her study sessions with the Cutie Mark Crusaders for some time, and has been remarkably pleased with their progress. And kind of worried, as they keep getting better and better. She might be a princess now, but TEACHING THE CUTIE MARK CRUSADERS? She'd rather not become their formal mentor. Yet.
A letter is sent out, and Princess Celestia gets a call for help. And, an opportunity is born. After all, why not? After a thousand years, perhaps it is time to do something else, just for a bit. She does have a rather wonderful younger sister who hasn't been ruling a land for a thousand years, and who has been studying how to rule for the past four rather intensely. So, teaching three fillies undercover? Sounds like fun.
And Luna. She's wanted to be as respected as her sister so much that she once went mad over it. But now, she's simply being handed the power that she once coveted for so long and no longer wants. What kind of a world does she live in now that her highest desire has been so casually granted?
With all of this, What Could Possibly Go Wrong?
Arcs:
Escape: When Celestia hears the call, she jumps at the chance to teach again, especially three fillies who might surpass her grandest student. But first she's got to escape her own identity...
Examination: So you want to find out what a filly does? Time to test her silly, so long as you don't taunt the Happy-Fun Filly.
Excitement: So, you've found the Happy-Fun Filly. Great, but the other two need help as well, and Scootaloo has this ~Great!~ idea for how to use her new-found magic...
Encouragement: Cometh the Bubbles, and a whole load of new headaches as they find out just what being a Happy-Fun Filly really means...
Exaltation: Where's Luna been this whole time? She can't be very happy about being left in the lurch holding Equestria. She could really use somepony to talk to.
Exercise: 'Sunbeam' is looking to teach the Happy-Fun Filly more, but first she needs to start taking care of the wagon-tire she keeps around her middle so that certain Unfun-Time Fillies don't interrupt her. And there's this big draft stallion looking for help...
Dat kuvver pik
Man this looks hilarious. Using 'plot' as a synonym for butt in-universe causes me pain but maybe it won't come up again. This is gonna be an incredibly wild ride.
Celestia is doomed.
...it shall be read.
...it has been read. It shall be followed.
Well seems interesting so far. Although Twi as their teacher makes more sense.
I do hope the speedier output will reduce the presence of the style I found so confusing in your other story.
Once again, your Tia-Luna dynamic is quick, witty, and cute as hell
Can't wait to see where this lands them. 
This looks promising. I'm following it!
Would this also be in the 'Scootaverse'?
--Spade
Just seeing the last five words of the description made me want to stay up and read this before I get that so-much-needed thing called sleep...
"How hard could it be?"
"What Could Possibly Go Wrong?"
Sentences like these makes me terrified of all that they forshadow
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Celestia: AAAAAAUUUGH! WHY IS MY EVERYTHING ON FIRE!?
Title and cover reminded me of this picture:
fc07.deviantart.net/fs71/i/2012/119/5/5/mlp_impossible_game_by_w1kk3d-d4y06wu.png
This is going to be.... interesting. Hopefully many shenanigans shall follow.
I can see the chaos, devastation, confusion, misunderstanding and tree saps from where I am currently sitting, and the plot didn't even manage to reach that far yet. It just the good.
I love the CMC they are my fave, and knowing tia will be there mentor I feel confident in saying she will still look beautiful bald when she rips out her mane in frustration.
1)The CMC being so awesome that they have TWO latent special talents instead of one? 100% Rainbow approved
2) Is Celestia related to Lucifer?
*Reads description*
Other than everything? The world might explode. Twice.
In other words, the CMC"s future is theirs to decide.
Eh, she took it as well as could be expected.
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She's....
Probably fine.
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As "fine" as Twilight ever gets, anyway.
To be fair, Twi has lost her shit before over far, far less.
I do hope the Cutie Mark Crusaders turn out to be as supremely powerful as Twilight Sparkle fears.
A few typos in this chapter:
elixers --> elixirs
tenants --> tenets
humms --> hums
rescend --> rescind
protegee --> protégé
millenia --> millennia
Sunbeam..? You don't mean...
subtlty --> subtlety
secoond --> second
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Empress Sunbeam Sparkle of the Phoenix Empire? No, certainly not!
4314402 Actually I meant Scootloo's mom. From Scootamom.
Is this in the same verse as Scootamom, for that matter?
You forgot to de-bold and de-italicise the things that are in the speech part but not part of the speech (Fluttershy's yipe and Twilight fainting).
Unless you did that on purpose, but I can't think of why you would...
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Ohhh right, I forgot that was her assumed name
That doesn't really sound like a name a pony would have
And Celestia's first act as a regular citizen?
.....Attending her Student's funeral.
when I saw the title I thought of these three:
glamgrid.com/wp-content/uploads/2014/01/Top-Gear-Experiment-%E2%80%93-What-Happens-to-a-Car-When-it-Gets-Hit-by-a-Train-2.jpg
4316166 Jonathan is an apple cultivar. It's a good apple for apple pies.
The opening letter feels like a bit of an info dump, but your prattle between Luna and Celestia is wonderful.
Mass hysteria ensues, conspiracy theories literally fly out the window, etc. etc. Just the usual stuff :3
Also, 121 likes and no dislikes so far! Nice! ^.^
Mayhap this has already been answered, but does this take place in the same universe as Scootamom?
Hmm, you are fusing quite a bit of comic canon. I like it. This is fave-worthy.
*Elsewhere*
Discord shot up in his seat on the wall, a shudder rippling through his body. Something was coming.
Though reformed, Discord remained the Spirit of Chaos and Disharmony. As such, he posessed a certain sense, a type of radar as it were, which allowed him to know of the approach of any large-scale chaos not caused by him. Pulling this radar out from under tail-scale number 253, Discord felt what may have been called nervicited except that nervicited wouldn't be strong enough of a word. It would be more like terristatic, yeah, terristatic would be more like it. He didn't know when it would come, nor from whence it would come, but Discord knew that chaos of potentially world-shattering intensity was coming, chaos that even he would do best to fear. Trembling, he briefly considered fleeing to one of the nearby realities. Dismissing the idea, Discord consulted his hardly-used schedule. Seeing otherwise nothing of interest scheduled for the next few centuries except for some upcoming visits with Fluttershy, Discord elected to stay for the show. It may be a horrible idea, but it would be quite exciting while it lasted.
Head-cannon accepted, and now all I can think of is medieval weaponry
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Think more "Paradise Lost"
I knew when you used the phrase "what could possibly go wrong" I had to read this. You're not doing too bad.
The maple syrup... IT'S EVERYWHERE!
Perfect princess weapon ~ icky slimey goopy stuff.... Love it!
Such silliness.
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Much Wow?
Lol!
Stupid guards.
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Chuckling while facepalming can be complicated.
Celestia is being oppressed! Come see the violence inherit in the system!
Welp, this is pretty damn great. I hope Celestia keeps using this disguise for the duration, because it's hilarious.
And is perfect for harassing Big Mac.
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*Luna, deadpan as she is hung from her ankles upside down and covered in maple syrup. And feathers.*
"Help. Help. I'm being oppressed."
Wow. Those guards are way too overprotective.