Classroom regions will be announced which region it is 30 minutes before the class. The region is selected ahead of time, and only will change if the server is restarted. Classrooms will allow users to enter the classroom whether a class is going or not. You will only join Class chat if you meet the requirements to become a classroom viewer that is being actively used (detailed above)
As of v1.3 you can not see the region when hovering over the class in /schedule and regions will be available for professor lessons or automated lessons alike.