Does your school have a counceler? I think you should go there because if you're feeling depressed, he/she can help you find the right kind of help. If it is affecting your schoolwork the school will send you to a therapist.

I am amazed your doctor and mother concluded there is nothing serious going on. Depression is heavy stuff. A doctor, of all people, should know that there does not have to be a reason for depression. Your life does not have to be tragic for you to feel the way you're feeling. Depression is a chemical inbalance in the brain. After you find a counceler, I strongly suggest you have him/her give your mom the facts of depression.

Good luck! I hope this helped and I hope you find the help you need.