It depends on what certificate you take. If it's CET-4 or CET-6, you can't skip a grade, that is, you can't take CET-6 until you pass CET-4 or CET-6. If not, you can't take CET-6. If it is CET-4, CET-6 or CET-8, you don't have to take the exam. If you haven't passed CET-4, you can take CET-8 directly, but CET-4 or CET-8 is an English major.