What Does It Means?

Faryal

"You are a grade ahead" means you are in the next class in your school?

Is it correct to say so?

Yes. But more often, "You are a grade ahead of me".

a grade ahead of Tom; a grade ahead of Mary; a grade ahead of you; a grade ahead of her; ...

The opposite is "a grade behind [me / you / ...]".

CJ

Faryal What does it means? mean?

After auxiliary do (do, does, did) you must use the base form of the verb.

CJ