While it may be true that the parents had a choice on where to send their kid (more than likely not most schools are "assigned"), it's also true that just because a rule was explained to them in the beginning, it does not follow that they MUST obey said rule.

People will follow rules until they "wake up" and find those rules interferring with their personal liberties.....

If a law banning the 9 is passed, how many people also give up their snakes obeying such a "rule?"

There can be rules in place, and we can follow them until we find them intolerable and then it's up to us and purely our option to continue following those rules or not (regardless of whether we knew of the rule in the beginning, we can always affect change).