> Does it (despite the name) take place in a conventional class room

I think usually yes, otherwise you call it something like a "spare" (i.e. a period where you don't have a class and aren't monitored).

I suppose "hall" probably traditionally refers to something like a "great hall" or "meeting hall" not a "hallway", i.e. a relatively large room.

In most circumstances (assuming it's during normal school hours and not an after-school program or something) it's in normal classrooms. I think there's some variation on whether students can't choose a classroom (IE "I need to work on or get help with Math") or it's always an assigned room (IE "my study period is always Ms. XYZ's room).