If it is a public school, the rules SHOULD be the same as local parks. If it is a private school... different rules apply. either way, check your local laws. If it is not listed, or prohibited, then go for it. However, if there is a local ordinance, or state law that prohibits it... stay away, or ask permission.
Either way, you should ask for permission, but more importantly, check your local laws. Ignorance of the law is no excuse, and while many say "what's the worst that could happen? I get a warning." there are things that are a lot worse. I posted this in a thread earlier this summer:
"It isn't worth it. Back in the beginning of April, I defended a gentleman who used a metal detector on a prohibited site (underwater). There was a statute that prevented it. Here was the total tab after all his gear was confiscated:
$2500 in Scuba Gear
$1000 for and Excalibur 1000 (bought about 2 months ago)
$3500 for 14' zodiac and motor
$1000 in fines
$750 legal fees (to me )
$8,500 Total bill"
Easier to beg for forgiveness than ask for permission... not always. I'm not saying that this will happen to you... My client didn't think it would, but it can.
steve