The fire alarm went off in one of my university exams. We all left, a bit confused as to where we should be going. Outside security guards with megaphones were telling everyone it was a real fire and that we were to move to the far side of the campus! That was when we realised there was so much confusion that we could cheat... except, no one could remember any of the questions, let alone the answers!
We just chatted instead for ages, then then went back in to finish the exam. They gave us 30 minutes extra time, which annoyed me -- since I'd already got stuck and could do no more!
My school had a fire when everyone was still walking into classrooms at the end of lunchtime. That was fun -- about 2 hours spent sitting on a (pedestrian zone) road outside! Or not. It was a pretty serious fire in the basement, and lots of the senior staff were clearly getting really panicky, which was funny since they were always so strict and organised for fire drills.
And my university caught fire (again) in November. I was just about to leave when the alarm went off at about 8.30pm, I thought it was certain to be a false alarm so I didn't bother packing up my stuff. After 50 minutes of standing outside I got back in, and left 1 minute later. We were going to go to the pub, but the security guard kept saying "just 5 more minutes!" so we didn't think it was worth it.
They don't have a bell/siren, but a loudspeaker that says "Emergency! For safety reasons, please evacuate the building."