I'm not sure if I understand the question, but when I hear "average student" I think about his/her school grades (a B/C student here in US grading system). If I hear "ordinary", I think there is nothing special about him compared to other students. He does his homework, shows up to class, etc. but doesn't do anything (good or bad) that makes him stand out compared to other students. If you aren't talking about students, I think you can use average or ordinary. An "average guy" and "ordinary guy" mean the same thing. "Did you do anything special today?" "No, it was an average / ordinary day."

谢啦~但是No sheet又是什么意思呢？　

No sheet, BR's spot on.

Put into Chinese they both literally means "普通的" in this instance. The slight difference would be: ordinary means he is just like everybody else, doing not too bad, neither very good, he doesn't things terribly wrong, neither things extraordinary, he obeys the school's rules etc. But saying an average student will probably particularly concern his studies, if he gets a great grade on every subject.