I have an idea which just might work. You will need to use some other valuable thing you've got as a bait. Use a high end Cell phone or some other gadget you may be having. I am pretty sure that the I-pod thief is one of your classmate and maybe even one of your friend. Now suppose you are using your newly bought mobile as a bait, cover the mobile with plastic cover and just before leaving the classroom spread some ink on the cover. Do this secretly. If you are lucky, the thief may have taken the bait and got his hand messed up in ink. After you return to your class, immediately check your bag, if the phone is missing, immediately inform your teacher and get permission to check everybody's hands, the one with the ink on his hand is your thief. Once you catch him/her, it won't be long before he owns up to stealing your I-pod. Now all this is extremely risky, you might lose another gadget because of this but it might work. This is the only way to catch the thief as I see it.
Best of Luck.
P.S : I think using a bait is a good idea but I am not too sure about ink (its messy), any other ideas to identify the thief once he has taken a bait?