The chart showing the bands (sadly a good quality version so we can't see the symbols of the bands properly) has sixteen entrants. Going by what we see in the film, the ranks include the following:

The Rainbooms
The Dazzlings
Trixie and the Illusions
Flash Drive
Lyra and Bon Bon
The (Cutie Mark) Crusaders
Bulk Biceps
Octavia
Snips and Snails
Eco Kids
Derpy's band
Possibly Diamond Tiara and Silver Spoon (we see them during the Battle of the Bands song and making a banner in the gym earlier, but we don't see them perform)

So that's four slots left open. Do you think any other established characters were possibly competing, or just the anonymous humans made for the films?

3675147 Probably established characters. Maybe even Vinyl?

3675147
I've been looking for a good picture of that chart ever since I saw the movie, just for this purpose.
You left off Photo Finish's band, so that's one more. But I think that the rest are probably just other random students. After all, pretty much everyone really wanted to win.

3675448
Probably not Vinyl, because she wasn't under the spell like everyone else. And she would have won if she was competing.
Though I think she's probably a member of the Rainbooms now, so I guess she did win.

Grabbed a screencap from the stream, we can clearly see all the band icons here.

Left side, top to bottom: Unsure, Trixie, Lyra and Bon Bon, Unsure, Snips and Snails, Bulk Biceps, The Rainbooms, Octavia.
Right side: Possibly Photo Finish, Possibly Diamond Tiara and Silver Spoon, Drama Kids (presumably), the Dazzlings, Eco Kids, Flash Drive, Derpy, CMC

Ah finally! I always knew Derpy was up against the CMCs (and I figure she lost), now I know for sure! :D

3711975 By proxy we can also presume that Flash beat whichever of them went through so he could lose to the Dazzlings in the semi-finals. As if we needed another reason to dislike him.

3712598
I think what we learn from a quick analysis is that Principal Celestia and Vice-Principal Luna are plebs.

I'm sure it's not their fault, though. They were under a mind-control spell, after all.

3712598 Probably the CMC. Not to mention that when he lost, he blamed Twilight Sparkle and the Rainbooms, even though he wasn't on that side of the chart. Flash acted like a total tool. The only excuse I can make for him is my headcanon that everyone was a little messed up by the events of the Fall Formal (my original idea was that no one could remember anything, but obviously that bit has been Jossed) and that Flash in particular was a nervous wreck. Still, I don't know how he expected to win, especially if he played one of the songs I had him write, like Bedroom Spackling Project.