if you just act confident no one will stop you. when i was at uni a guy I knew left school.. and left his bike locked to the rack - right near the security box at the entrance to the school. it sat there for a month until i thought: damn its just gonna rot away there. so I borrowed some giant bolt cutters from the scene shop - about 3 foot long and red. walked down there past security.. popped the lock, and then walked back with bolt cutters and bike on shoulder (someone had already stolen the wheels).
they didn't say a word.
fixed up the bike and then left it there unlocked with a sign saying "free bike" when i left school.