Well, it depends. I usually manage to keep my clothes quite clean. In a week of school (5 days) I'll wear:
5 shirts (One for each day)
5 pairs of boxers (One for each day)
5 pairs of socks (One for each day)
1 Jumper (I don't get it that dirty)
1 pair of trousers (I don't get them that dirty, sometimes a tiny fleck of mud, etc.)
So, basically, I change my underwear daily, but I'll wear outer layers a few times unless they're dirty.