How to become a good soccer midfielder?

Ask them if they can test you on attack, midfielder. Also you should train your speed, passing, acceleration, shooting and if they see you got good speed, shooting, passing and eaven beating players with skills or dribbles they might put you on attack to. If you won't ask them they will never put you on the position unless there would be new defender in your place or some midlfielder or attacker got injured. Also you should watch some videos how to play on these position you want that helps alot. Also if you play bad as defender they might change you as there would be risk for the team that you might do mistakes and that kind of stuff then they would check what position else positon you can play. Also it sounds that your school team is not profesional and do they have trainer/coach or something? if not then yeah you would have no chance to get in better position becous trainer/coach decides everything and test every player what they can do.