Knock off Danvers and the 2 Prefects, then you have about an hour on the clock before they re-spawn (about 6 minutes real time) to go do your mayhem on the second floor. No matter what, though, once you leave the office Danvers will re-spawn behind the counter.
Just watch the clock and get back to the office and back on the counter before time's up.
Sometimes 3 or even all 4 Prefects will be on the floor. This happens rarely. At 7 PM, they will all re-spawn, no matter what, so watch for that.