This is a "File Case".
It's Sonic Adventure 2 Battle themed, and made of plastic. You're supposed to put your school paperwork in here to keep it safe from bends/tears. Notice the cut-out on the one side so you can grab the papers. It's different than a folder for your binder, since it has no holes for a 3-ring, or otherwise. Having different designs for different classes would keep things organized.
The design here is great! It's a colorful montage of all the characters they could fit including Dark, Hero & Neutral Chao, Sonic, Knuckles, Tails, Shadow, Rouge, Amy & Eggman there in the corner. You can see the SonicTeam & Sega copyrights at the bottom. This is a super folder/paper holder to collect...it's too bad that it's uncommon.