I'm in the exact same boat as you, expect my GCSEs are coming up soon and I just can't bring myself to revise or do anything, my depression has gotten a lot worse over the past few months, most of the time I just come home from school and sleep because I don't want to do anything else.
I would really suggest trying to see a counsellor or even your GP, they can refer you to someone and it could help you manage exams and studying rather then letting it get on top of you and ultimately failing. I would also suggest telling someone at your school about how you feel, they should help with it then and they'll know the reasons why you attendance and grades might be dropping. I told my school recently and they're gradually helping me manage school work I feel stressed about, I do feel a lot more relieved now I've told someone so I think that's really the first step, to tell someone about how you feel.