They let you play on it? Nice. 8 YO DD and I went to the local HS complex this summer to practice and about 30 minutes in the HS coach came charging in telling us we cannot play on the field without permission. Must have cameras there or something. Field wasn't locked and had no signs saying not to play on it. She acted like we were 2 teenagers getting ready to graffiti the place..was about ready to tell her that if she wanted her team to win some games once in a while she might want to let kids practice on it..but amazingly I was able to bite my tongue..
If I am not mistaken, unless it is a private school, HS facilities are paid for with tax money. Therefore everyone in that district should be allowed to use the facilities, within reason. All you should have to do is contact the proper person and confirm that. I know that when I was in school we had a parent that through a fit because they could not use the track when they wanted and demanded a key to get in. AD said ok you don't get a key but we will unlock it for you whenever you want in. They got quiet.Then after a few more request they simply left it open 24/7.