From my experience-
Art students= bad. Put lots of holes in the walls. Friend always has a dog that stays over. Cleaning is not part of the mindset. Collect lots of crap.
Female students = bad. Lots of mess in the bathroom and spills on the floor/clogged toilets. Not always as clean as one may assume. Fill washing machine to the brim and leave clothes in it for days. Leave clothes on line for days. Don't waste money on washing powder/toilet paper/cleaning products (need to save to go to Bali). Fill bath to the brim and stay there for extended periods of time. Never turn off lights.
Male students = bad. Bring mates home for a drink that turns into a free-for-all. Room always stinks - rarely wash clothes. Never clean up vomit.
Asian students = bad. Like to cook but not clean up. Chicken legs, fat, chopsticks...all down the sink. Toilet never cleaned. Lots of visiting (ie homeless) friends. Lots of food/eating in the room. Lots of downloads for friends - sleep all day but talk all night on skype. Never turn off lights - or anything else. Hang washing out the window.
First year uni students = very bad. Not old enough to be responsible but old enough to be a real pest. Think its great that boyfriend/girlfriend can stay over as much as they like -for free. Parents don't want to know. Relationship breakdowns become a 'whole-house' issue.
Medical students = only have experience with nurses - all of the above applies.
Noone believes they used 'that much' electricity.
Hope this has helped - perhaps other people have more positive stories to share
Have had experience with two differnt universities - same story for both.