Help me get out of this situation?

So we have this thing called micro at my school where we create our own businesses and use fake money to buy real food to make our product. So on my day off a friend and I went to the warehouse where we get our supplies and bought icing for the next day. We told them we didn't need it right now but they still gave it to us( half empty not full that we already owned) and refused to take it back. So my buddy puts it in his shirt to take it back when micro ends. Towards the end of micro we find out someone smeared chocolate icing in the elevator, and the manager of the warehouse has the can that was in the elevator with the icing. They pull us up and lecture us for a bit and my friend gives her the can of icing that WE had. So we come to find out that one of us or both im not sure is going to be written up for theft of school property. How can they do that when; a) we bought the product b) they handed it to us willingly c) stealing is without paying, and without consent of the owner. Please help because this could ruin my chances for highschool.