From the ACLU of Georgia website, see Q/A below.
Once again, most of these rallies are private events (see the message from Valdosta State's interim president). Even though the building/arena might be located on public-school property, they're still rented as private events.
But hey --- if you want to think I'm wrong (and the ACLU, and the Valdosta State president, and the UIC Chancellor, and Jeffrey Toobin, and the Senior Judicial Analyst for Fox News), have fun.
Q: What about private property?
A: When it comes to private, as opposed to public property, First Amendment protections are much more limited. Private property owners have the right to control access to their property and may require people to leave at their discretion. If persons engage in speech activities on private property against the wishes of the owner, they may be subject to arrest and prosecution. It is advisable to obtain permission from the property owner prior to engaging in activity on private property. Please check with the management of a private facility prior to engaging in free speech activities at those locations.