Seems like mostly what teens will do these days is steal! If you have iCloud running on your devices, please go to http://www.icloud.com. When the page loads, please enter your Apple ID (if you use iTunes, App Store, iMessage, you do have one) then enter your password. Once everything is loading, you will see a icon that says "Find my iPhone". Click that app, and it will search for your iPod touch. You can lock the device, add a passcode so it can stay locked until you find it or get it back, erase the iPod, and play a sound.
When you get to school the next day, which will probably be tomorrow, go tell the principal or the teacher. Make sure you check your locker or desk very good before going to the principal or teacher to be sure! They can help you get everything situated, and if you have the iPod touch box that it came in, bring that as evidence so if the person lies saying that it was their iPod touch, you can pull out the box and prove them wrong!
If you have any more questions, feel free to leave another message.
Hope you recieve your iPod touch back! Next time, keep it in your pocket or in your locker or hide it very good in your desk!