Its really not hard to get permission, if you ask politely and say please they most offten will give you the go ahead. Just tell them that you would do it at night so you would not hurt any pedestrians, also if they bring up acountability tell them you will not sue them if you get hurt (dont bring this up unless they bring it up first or you are getting desperate). Also dont tell them the exact time you are going to do it at first, that way if you dont get permission you can just come 2-3 hours later. If it is public property you should be fine, especialy schools in the summer.