Schools are different all over the world. I believe rooms were assigned to teachers as much as possible, but not every teacher needed a room all day every day so there was the occasional change-up. For instance, sometimes biology/chemistry/physics needing the few lab rooms for experiments, and those had to be scheduled in somehow. That also caused other rooms to bump every now and then.

Generally, a teacher not showing up was no real reason for skipping class (no matter how much urban myths said otherwise) but if the absence was known in advance we'd get a free hour. Most kids used those for some extra homework time or to just hang out, unless the free hour lined up with the start or the end of the day.

We didn't really have study hall, just the normal areas in school that you'd also hang out in during breaks (or holes in the schedule, as older kids with personal schedules occasionally had).

In university this rarely happened, most people would leave after about 30 minutes because a lot of them traveled half an hour or more to get there in the first place. People were a lot less willing to abandon classes once they were paying out of their own pocket for them.