Uh...lockers. I think most schools have them now, but they're not a big thing like in the States (see, I watch Buffy, I know stuff ). I reckon you'd make friends really easily. Obviously, the language might be a barrier though (only kidding!). When I was at school, it was pretty friendly and people were so curious about foreigners that they ended up being the most popular people!
Vegetarianism has been so popular here for so long that I doubt you'll have problems with your IBS-diet. People are used to teenagers being "weird" . I find eating in this country easier than anywhere else I've been. You can get soy and dairy-free products at the supermarkets and it's rare to find a restaurant that can't do me anything. I've even found that pizza restaurants will do me a yummy pizza without cheese - just ask. I usually just have the problem of stopping myself eating too much!
Only thing would be to stock up on fibre supplements and Beano before you come over cos we don't have them!