what should i do? please i need legal help!?

ok so at school there are these kids that i used to hang out with at lunch in the library. well i was playing a card game called yugioh(please dont judge me. playing yugioh is not the point of this question.) and while i was playing they somehow stole my yugioh binder with all of the cards in it. well the binder was worth over 1000 dollars and i didnt know what to do at the time. and at the time i didnt know the binder was stolen until i got home. well this weekend i went to my local tournament and i saw them there with my 1000 dollar binder. i asked them where they found it and they said that a "guy" gave it to them. well i know that its my stuff because i have a special mark on the back of the sleeves that i put the cards in. and all three of them were there when i suspect my binder was stolen. when i asked for it back they said no. i tried to get my to help me get it back but they still wouldnt give it up. they said that it was theres and that i had no way to prove that it was mine. after i told them i was going to get the cops involved if they didnt they left. now i ask you not to judge but ask for help. what should i do? how do i get my stuff back? my school does have security cameras but i dont know where to go. and how do i get my stuff back legally? and i still have 2 more weeks of school left.