I hunt schools more than anything else... I have never sought permission. Most schools have signs on the proprty stating the rules. Typically I see "No dogs," "No golfing," "Drug free zone," "No guns," etc. I have yet to see any signs that say anything about metal detecting. That being said I take excellent care of the property, cleaning up my holes nicely and carrying around a bag with the junk I've picked up. And the posts below are right, you'd have to be nuts to detect while school is in session. Early weekend mornings are the best; or during the day when it's cold and/or a little rainy.