I have failed two grades and I'm 15 in the 8th grade where do I go to try to skip up to my right grade ?

Have you talked with a teacher about this?

How are you doing in school now?

So you are now a straight A student, working at two years ahead of your current class.

But the real problem in high school is that most require a certain number of credits to proceed from one grade to another and they will require a total number of credits to granduate. So at this point it is unlikely that you will be able to.

Go to your school oficce. Ask them how do you skip a grade. Mostly if you want to skip a grade you should be a good student and you ask for extra homework and help at home. And tell your principal how good you are doing. If he says you can skip 8th you go to your school disterk. And take a test that tells you you know most stuff in 8th grade. Wo-hoo you just skipped a grade.