室 is 'arrive' 至 under a roof 宀 — the room is where you arrive and stop. In ancient houses the 室 was the innermost chamber, the destination at the end of the courtyard sequence.
教室 classroom, 办公室 office, 卧室 bedroom, 浴室 bathroom — modern life is a corridor of 室s. The old phrase 登堂入室 'ascend the hall, enter the chamber' still means mastering a discipline to its inner room.
Under the roof 宀, you finally ARRIVE 至 — welcome to the ROOM.
In Chinese schools, students stay in one 教室 and the teachers rotate — the classroom is the class's home turf, decorated, cleaned, and defended by its students.