bossmantx, with all due respect to curlysgold's advise to "go ask the principal", read the "ban detecting" thread that is near this one, right now. See the similarities?
What things have you researched (Ie.: what have you read) that makes you think that San Antonio is a "tough town to metal detect in"?
Ask yourself this: do other people access the school grounds after school hours? Ie.: persons who want to jog the track, use the hoops, fly a kite, etc...? If so, then why do you think you need permission, anymoreso than them? Unless there were a specific prohibition, that is. Rules are usually posted on the sign at the school turn-style yard entrance: no dogs, no weapons, no alcahol, etc...... If you see nothing there prohibiting your activity, and you plan to leave no trace of your presence, then ......... you get the point. I detect school yards in any city I come to, wherever I travel, and don't have a problem. Is that to say that if I asked "enough principals" that someone might say "no?" SURE! But isn't that a little like asking if you can wear a tree-bark suit on the lawn and whistle dixie? Authority would probably also say "no" to that too. But if you just went and did it, did anyone really care? probably not.
