These are always good tips: http://talk.collegeconfidential.com/high-school-life/2026961-what-to-do-when-you-arent-doing-well-in-a-hs-class-p1.html
Go to your GC or school Psychologist and talk to them. Talk about the accusation. Talk about the fact you feel depressed.
Ask your parents to take you to the doctor to get evaluated for depression.