Can a High School ban a person?

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My friend has been banned from stepping foot on a high school campus, yet he has done nothing wrong. He is not a risk to society, not charged with a felony, not a predator. He used to teach at this high school for three years until they fired him. I'm working with a theater company and the high school has let us use their theatre. When word got out that my friend was going to be a stage manager for our production somebody went to the principal of the high school, and my friend has gotten banned from helping us. Is this even legal? I would understand banning him if there was a just cause and reason, but there is none. He's not a threat what-so-ever. The only connection is that this play is taking place on the high school grounds. None of his former students are in our production and he has no intention of getting his teaching position back. None of this makes sense to me and I wonder if it makes sense to any mylotters.