How to eliminate stress in my life?

I'm really stressed right now. I come home every day and just cry for no reason. It's mainly because of my geometry class which I'm failing and the teacher treats me horribly. When I get stressed I just take a deep breath but that only works for a few minutes. How can I eliminate stress from my life? I heard it is toxic for your body and I'm starting to get stomach aches often from so much stress. Much help is needed, thanks :)