Is this what's making me lose weight?

It certainly could be, it also depends on what you're eating. Make sure its the right stuff, and often enough in the day, you need to consume about 1800-2000 calories a day with exercise. You might also want to think about any causes of stress at school, if you are worried about anything or anyone, you need to talk to a teacher and your parents to put things straight. Try not to worry too much, everything can be sorted out one way or another, and the more you share your problem with those you trust, the better the chance you will get through it. Good luck, and don't worry, be healthy!