I would say no too though I know it's easier said than done. I have a "no food or drink from home" policy and some still drop their kids off bright and early with a bag full of candy or a cup full of juice
I hate confrontation so I stick it in a baggie and set it on the shelf for parents to pick up with their kid. I explain to the kids that it's not fair to the others if one brings something and the others can't have any. Hopefully they repeat that statement to their parents.