If the school allows you to take part, it does not dismiss you from school, it becomes a school sanctioned (sponsored) event. Are you really this daft?
Ok, so imagine the kids are sitting in a classroom, discussing, say politics. If a kid brings up the issue of legalisation of cannabis, and argues that if legalisation doesn't occur, kids will do it anyway, (ie advocates use) and the school doesn't take action against the kid, they therefore take on that opinion, it becomes their own and thus they have sponsored it?
An appropriate action for the principle would be told hold a debate with the child on the nature of cannabis use. If they wanted to change the opinion of the child with the banner, they should explain why they object, not simply take sanctions against them. That just makes people defensive, something we recognise on these boards, and entrenches them into their perspective. Doing this would also negate the need for a totalitarian limitation of free speech.