Night life in Durham is good - you have to remember that although it's a small city, it does cater to thousands of students descending on it! In addition to loads of pubs, clubs, and bars, there are also the 16 colleges which regularly put on events and all have their own bars and activities to get involved in (you can go to bars in other colleges except on specific nights). You won't get bored I promise you. Even if you do want to have a big city for a change every now and then, Newcastle is only a 15 minute train ride away.
There is a variety of people from different backgrounds as there is at any University. I myself came from a comprehensive school in my home town, and I never felt out of place. Sure there are some posh people there (called "Rahs"), but there will be just as many people you'll be able to relate to easily. I can't recommend Durham highly enough - you'll love it there