Girls especially like it when you give something that shows you had her particularly in mind. They don't want something useful, or something popular (unless it is a coach purse or something -.-), but they DO like it when you remembered something about them. I always remember when my girlfriend talks about something she had her eye on, or something she likes in general, such as how some necklaces are too large, some are too small, and I'm there to compare what size she is really looking for, and then I find the perfect one and get her it when she doesn't expect it. Or, I remembered that she likes pigs, and so I'll just get her random little trinkets as I see them, of pigs.

I don't know really what guys are supposed to get for girls at 15, as I didn't start dating until I was 21, but I'd say just go with whatever you consider to be right. Do you want her to know something specific? Like, do you want her to know you want to strengthen the relationship? Are you unsure right now (I'm sure you are), and so just want to let her know you are interested in her still? Really, it also depends on how long you've been going out, and if you think she'd be embarrassed with certain things at school. Just go with your instincts.  

If you are a bit tight on cash, especially with being 15, then perhaps you could ask your parents for a few dollars to get her something cute like a stuffed animal that you know she'd like? Or you could just write her a poem, or something. Be sincere, be a gentleman, and it'll all go well. She will love it no matter what :)