Yes, it is possible to score above 70% at North Bengal University. I have known people from the university who have scored above 70% and even 80% in their subjects. But it also depends on your subject. If it’s a science subject like Physics, Mathematics, or Computer Science that's achievable. But it’s a little tough for Arts subjects. 

Also, recently NBU started a semester pattern and study. In the semester system, the question pattern was updated and it makes scoring above 80% easy.