Should I stand up to a school coach?
Maybe. Why would you stand up to a school coach?
Because the coach won't let me leave practice early
You have an obligation to complete practice and be there for the allotted time. It is your responsibility to complete your practices so you need to stay for the entire timeframe.
It's not fair that I have to stay the whole time. I want to leave practice early but the coach won't let me.
Your coach makes the rules so you have to follow his authority. Otherwise you might get kick out the team.