I know exactly how you feel. I can really tell that you are a very intelligent girl. This is extremely hard, especially when you have morons like your teacher around.
Really I can empathize with you because I know how you feel. I have two children one of which is eight months old. I'm trying to exercise and diet and I'm a full time student. I can tell you that you will run in to teachers like this the rest of your life. I have teaches that always make fun of accounting majors (I'm in business school and I'm an accounting major). High school is tough just make try and make it through.
As far as exercise goes I just do it. I am pretty sure they give lunch brakes. Most schools have a track. You could walk around that on your lunch brake. Don't worry what people think either. When you are in high school it seems like the social aspect is so important. Really though when you get older you will see that all the worry is for nothing. All the way through school I ate lunch by myself and had no friends. I made it through and I know you can too.
Hopefully I can fit in here. There seems to be no other place for me on the 3fc forum.