Interesting question, and a lot of interesting responses.
I think that for most people, college (I not sure what that equates to in British terms) or university is part of a normal life.
I went to university, got my honours, then went back to do a postgraduate diploma, and I did some stupid stuff when I was an undergraduate. No lasting damage though, and my faith was strengthened by what I saw, experienced and learned about the world.
The vast majority of us are going to have to work at some point in our lives. Going to work with unbelieving colleagues and doing your CDP with all sorts of people isn't much different to studying alongside unbelieving students at college. It's part of the experiences of our wilderness pathway.
One thing I will say though is that it isn't good for a Christian to live in the same flat or apartment as non-Christians while at college. It brings all sorts of complications and moral questionings. It can be very tempting when your flatmate invites you to go to a barbecue in the park on a Lord's Day, instead of doing what you should be doing. Similarly, co-habiting with people who don't share your principles and moral values can be disconcerting. Your flatmate goes out for a big night and brings back a girl / boy for... well, you know what I'm talking about. Awkward!