You do not need to be a high schooler, or at least I don't think you do. If hero says it's alright you prolly don't even need to be at a school at all. Not everyone goes to a school it's just kinda what became the controlling power for the country.
B-Power is basically the name that they call a person's skills. For instance if the hole made you able to move faster or see behing your head then that would be a B-Power. They can be something as unnoticeable as changing your hair color or they can be life crippling forcing you to use a wheelchair the rest of your life.
Yeah those are most of the main factions, like hero said there are a few he didn't put in but those are for a good reason I assure you of that. You don't have to if you don't want to. Or at least I don't think so, if it's alright with hero there are actually independant states you might be able to join or you could just be a rebel or someone who has decided not to help your nation at all.
Not sure I can help with this last one.