What happens if i cant make it to a N.Y.C regents?

My earth science test is tomorrow and im taking it as a walkin, meaning i dont have the actual cl, ive tried to study for it but im guaranteed to fail, So ive taken the Lab practical already so if i dont go is it going to be a 0, or count as an absent, or is it just going to be my lab grade, Can i reschedule?