However, nor does the principal's convenience define "trespassing." Was there a set time limit? A written policy? Did he attempt to stay beyond the time the building is available for use?
Police should ask a few questions before whipping out the cuffs.
eno, I'm fairly well running out of gas on this subject. But I think if you are asked to leave a property and you are still there after two hours of negotiating with the police, they have every right to cart you off for trespass.