Well, the first thing that sprang to my mind was good old fashioned ear plugs; but those wouldn't really be practical in the school setting.
If you're really desperate, I'd recommend looking into the practice of meditation. It's given a bad hype by its spiritual association, but the core practice is to find a nice quiet place in your mind where you can shut out the outside world. Best of all, it can be done simply by focusing on your breathing. Whether your school knows about your sound sensitivity or not, no one will question you taking a few extra deep breaths each day.
Look into it: you'll either have found a good solution, or wasted 2 minutes reading this.
I have a touch-texture sensitivity (touching certain fabrics causes the sensation of fire) that's pretty hard to manage, especially because I'm still covert about my diagnosis. I get around it by wearing a bandana, and using it as a makeshift glove if I have to pick up something bad. That really doesnt help you, but the aforementioned suggestion of meditation might.
Good Luck, if you need resources, pst me and I'll help you out.