Are there any laws that prevent sex offenders from being on school grounds, within the school, at any time?

I need to know if sex offenders, particularly convicted child sex offenders, are allowed at all, at any time onto school property, inside the school or even anywhere outside the school. Our school scanned a person’s ID and the sex offender was let into the school to view his step children’s play in the evening. The person was not supervised or watched. This person was convicted, served time, and is now discharged. This upsets me and angers me very much. The principle is angry at me because I know the true details about that day and has told me that I was no longer welcome to volunteer even though I have done nothing wrong, just that I know the truth and was trying to tell others to be more aware.