To be honest, being involved in a sport in your school may be good for the experience and the chance to make friends. However you are already doing a sport the school does not offer. Only if you weren't active at all would I as a parent insist that you did school activities.
No, it is not. Many successful people were not involved in sports at all. It is great your involved in sports outside of school too and also projects inside of school. If your Mom is giving you a hard time about it sit her down and talk with her. Make sure not to get upset or sound whiny and just workout a plan with her that she is comfortable with too. She will be happy that you were considerate of her and you will seem responsible.
It's not necessary but it's healthy for ur body and if you're good at it you can even win scholarships. But remember to always of what u love and if u don't want to play sports then don't.
You have to play Because you know, you are managing all things and you deserve to be a good sports man.
Don't do anything you wouldn't enjoy. It will seem like you are wasting your high school career away if you do