It depends on the educational program that the kids are in. In my school, the teachers were very flexible and passed everyone... no matter what. Some teachers don't challenge their deaf students as they challenge their hearing students. If a hearing student was given an assignment to answer 20 questions for math and that hearing student complains that it's too much... the teacher will say, "too bad... work harder." If it was a deaf student, they would pity him and give him less homework. So, what happens? You've got a deaf kid going twice slower than a hearing kid. Most schools start their academic programs in the 2nd grade or 3rd grade. That's about 10 grades total. If a deaf kid goes slow, then he will be in 4th or 5th grade math by the time he's a senior in high school. Yes, I've seen this happen while I was growing up. Yet, I wasn't part of that group. I grew up mainstream. In fact, I was with the majority of the hearing students and at the same level as everyone else.
Another reason would be the use of ASL and SEE.