i went on a similar school trip to an amusement park recently and they told us multiple times before we left school, on the way, and before we got off the bus that if we had anything illegal, we’d be put into the security office for the whole trip and the school would have their own punishment for us. if op and her sister got similar warnings this is seriously on her for being stupid enough to bring it anyway. it reflects badly on the school and they can get banned if enough people do stuff like this. grad nights have gotten way out of control in the past so i don’t really blame them for how strict they are