I distinctly remember several parents being "Banned" from attending any school events after they were caught drinking at a dance they were supposed to be supervising. They were well over 21 so it was legal for them but alas it made no difference.
The difference is there is no preemption when it comes to alcohol. The school may make it's own policy in this regard. Same when it comes to RV's. Again, these scenarios have no relevancy to the OP as the state of Michigan does not preempt schools from regulating these activities.
Hells, who is your attorney? IANAL, but I personally feel both your attorney and the officer was wrong about trespassing you.
Officer: "You will be trespassed."
You: "Why?"
Officer: "Because you have a gun"
End of story
If you had entered a classroom and truly disrupted the learning environment in that classroom, then that is a different story. The speed at which the officer responded, or lack thereof, demonstrated how much you were not a disruption. IMO, there is no case for trespassing here.