You might ask around at school or in the school office. They may know of someone locally who does that and specifically does things with your school colors and logo.
You might also ask at your local fabric store. They will also know of people who might do this--often their employees will do that on the side to supplement their income.
If you can't find someone locally you might try www.etsy.com. If you aren't familliar with it, it is an online "craft shop". There are lots of sellers on it and you can do a search for whatever you are looking for. Do a search for spirit wear or embellished jeans and see what comes up. Find a "shop" where you like the items and then contact them and see if they do custom work. Most of them do.