I agree with every one else. You will not see any drugs- in fact, the closest i came to that was when I was with a group of friends and one had a spliff. That's all the drugs you will come across.
Smoking, too, is fairly low-key. Most people smoke in their rooms, or sit on the steps outside and smoke and so on. It can be fairly sociable, believe it or not, and it can be a good laugh to sit down as well. As long as you're not worried by passive smoking- even then, it's easy to stay away.
Perhaps the biggest of the three you come across will be drink. I live (in term time) in a university town, and so drunk students are a common sight. People I know do drink, and when out clubbing (which i did twoce all of last term) it is actualy an advantage not to drink. I havn't been drunk yet at uni- never gone beyond two pints- and on nights out just drink coke and water. In fact, if you are out it is good because you can keep an eye on friends who are drinking, make sure they have waterand so on, that they get back home, and so on. Just being able to look out for people is rather a nice feeling!
Anyway, the kind of socialising you described- as in sitting around with DVDs and so on- is actually quite common. It is far more usual for students (where I am at least) to hang around on their staircase and talk, gather in someone's room, and so on, then it is for them to go out.