I'm 21 and I've lived in Hull all my life, I even go to Hull Uni.
Of course, like any city whatsoever there are bad parts, and can be a bit of "violence" on a typical night out. But, if you're a student, you'll spend your time going to uni, which is in a very lovely place, lots of trees, near boutiques, independent coffee shops, etc and most likely live near by. The Lawns (uni halls) is in a lovely village which is about 10 minutes bus ride away, surrounded by big houses and again, very lovely.
As for nights out, there is Asylum, the student union pub, and of course, Spiders (an absolute must for any student; very cheap and friendly!)
Hope this helps