>>515381103
college janny here, I like to go on roofs so am familiar with many roof accesses. Some are just a padlocked hatch at the top of a ladder, so bolt cutters anon was right there. Others are actual keyed doors, and the keys have seven tumblers so only a pro would be able to pick them. At my work we have one grand master key that unlocks almost everything, but we have to turn it into secure storage at the end of every shift, and shit hits the fan if one gets lost. At a lower security level is a building master, which lets you into most things in one building. Teachers and workers can get a key that works in their office, the suite entry and the building exterior doors. Dorms have their own key profile and the masters don't even fit in the lock.
I don't really see how you can acquire a roof key without being part of the support staff. However, security issues like doors easy to break into get varying levels of attention, and a roof door would be low priority. Even a building exterior door might be malfunctioning and yank-openable for months before being fixed. My bet's the shooter either knew about a security failure like a broken door lock or brought a bolt cutter for a padlock. Someone who works at the college would likely be too comfy to take a shot at a guest speaker.