i'm not going to comment on
a) the use of the word parkour as a verb or
b) tresspassing on rooftops and the legality of such
seeings as those things have been thoroughly discussed on this thread already
To answer the original question-->
when i go out in my community, the public park has a lot of things i can use for "practicing" parkour--there is playground equiptment, rails leading up to different buildings, low fences around grassy areas, etc. etc. etc. It's a great place to work on a lot of parkour technique.
As for the school--my high school is a great place to mess around, work on technique and vaults and such on ground level. Technically speaking, it's private property, and if someone (janitor, security guard, etc) asks you to leave, you should do so respectfully and quietly, as not to give parkour a bad name or make other traceurs look bad.
I would also suggest practicing rolls or precision jumps in your back/front yard: when i can't get out into the community, i work with what I have at home.
The best way to find new places to get out is to just take a jog around the town-vault over any objects that get in your way, look around, see what you have to work with, and have fun!