this is a matter for state and/or local law.
of course, you could either visit the schol in person one day or call the school/school board and ask.
in some states, they are open to the public on an “own risk” basis, in others they are closed because of liability insurance issues.
GL
If it’s a district or county school it’s public property. If there are no prohibitive signs and you don’t need to jump a fence or break a chain or open a gate you should be okay as long as you don’t hold a rave or a soccer match there.