Correct. That is the reason why. Disney has a policy (at least 6 years) of no outside food/drink to be taken through the Grad Nite security to prevent any drugs or alcohol being taken in by students. Now, this year, oddly enough, there were a couple of chaperones who managed to get a few snack items approved by CMs as we were going through bag check. It was totally by accident, we had some cheese and crackers and a Reese's PB cup and the CM checking our bags, said it was fine even though we said they could throw it away. Now I don't know if it was because we were chaperones and not kids, or if they are now allowing prepackaged foods for everyone, but that was a change this year from previous but I wouldn't rely on it to be the same for nights going forward.
@Reyan88mph The Grad Nite coordinator at your school should have the rules of what you are allowed and not allowed to bring in. We received that information months ago.