Basically any character is acceptable as long as you're not apping a mass murderer as a teacher or something; having been experimented on is legit generally speaking (less so for students than adults, but still legit), but generally our approach to hecka overpowered characters is just ... power them down some, especially if they're going to be a student.

We don't actually mind what the source/s of the character's powers are in canon. It's an AU, so it's pretty easy to just pare powers down and attribute them to mutations, regardless of canon. For instance! I play Power Girl here, and in canon she has like a billionty one powers and is from space and shit, but to make things less complicated and her less overpowered, in XIverse she's a mutant instead of an alien and she's only a Flying Brick instead of having eye lasers and ice breath and super speed &c. &c. &c. as well – it makes my life like eight times easier.

If you really want to keep the experimentation in their backgrounds, we're not going to stop you, but if you're concerned about them, it'd be the work of like three seconds to make Apollo's sunlight-absorbing thing, for example, a mutant power instead of the result of being experimented on. But, yeah, if you do feel they're on the overpowered side we would recommend toning their powers down the same way we'd recommend it for the average over-the-top shounen manga hero.