I'm really sorry you had such a bad day..
Do you think it's possible having detention for beating those other students up was the reason you weren't picked for the solo though? That's probably, at least part of the reason you weren't chosen.
I'm actually kind of shocked you only got detention for that; If you went to any of the schools I went to while I still was in school you'd have been either expelled, suspended for at least a semester/quarter, with the extreme possibility of even having been picked up by the authorities(and your parents would have to actually pay for them to release you into their custody, or you might be kept overnight/for several nights before your parents were even given the choice to pay for them to release you) for beating other students up like that, two would definitely result in expulsion and being picked up by the cops.
For the band solo part of this, whether or not detention played a part in you not being chosen, other students also could of been practicing at home, I highly doubt you were the only one who practiced for more then an hour, some of your classmates probably practiced a ton, at home, on there own time because they also wanted to get chosen for the solo.
I do agree with Ven, you're not always going to get picked, no one is always picked every single time in their life.
It's also definitely not going to be your last chance, you're still pretty young.
Just keep working harder and keep practicing, whether you keep up with it at school, or find classes to take out of school.
- Not all those who wander are lost ~ J. R. R. Tolkien