There is no form of effective protest of which is “acceptable”. Protests are meant to disrupt, to get as much attention as possible in order to spread a message and bring action, and those who oppose the protest will always object to that no matter what. That’s the point.
For protesters to act so socked that they’re getting arrested for protesting against the power elites at their schools while on private property seems pretty naive to me. They should have known that this would be the eventual outcome and to have been prepared for it. Not to say it should have stopped the protests from happening— just that, for protests to be effective, it has to piss off the right people, and you’ll likely get arrested, free speech be damned.