i go after hours or weekends and leave by dusk. i always enter via the public entrances to the playgrounds. never had a problem. had the custodians come out and talk to me a little bit out of curiosity but never been kicked out or hassled.
I go all the time to the local schools. As re-tek said, don't go when kids are there, don't stay after dark and always take the trash out with you. You should have no problems. However; when I was at Lake of the Ozarks recently the schools there all had big fences around them with locked gates and you couldn't get onto the property.
So as long as they are not locked and no sign prohibbiting it have a great time.