What you have here is definitely in 3rd NF. Just a remark concerning what you are saying about scores: you should not miss the fact that each class is associated to 1 or many exams, so you have typically a one-to-many relation between class and exams (could be 0 to many for classes without exams?).
Then, you can consider that a score is held by a relation\table between the 'studentClass' table and the 'Exams' table. Depending on your business rules, a 'studentClass' record can be linked to one or multiple scores for a specific exam, depending if you can pass the exam once or multiple times.