I went to my school's orientation and they assigned a locker for me, homeroom, etc. So I got my locker and opened it and all of these supplies were in there (all the supplies that were needed). Im confused because they gave us all the supplies we needed and also mailed out a supply list.I don't know if everyone else got supplies in their lockers but my brother, who is attending the same school, didn't.

Yes, they may have accidentally double assigned the locker or used it for a demonstration during the summer.

First you need to be sure if others have got it or not. If not, You can alert the authorities

Maybe they gave you and someone else the same locker. You should definitely ask what's up.

Somebody obviously mistaken placed his/her supplies in the wrong locker. If you're a thief, lucky you. If you're not a thief, report it to the office. And if you do that, you'll probably make all kinds of points plus a friend. Lucky, lucky you.