The room has a hard limit. Turning somebody away who booked is far worse than an empty mat, so capacity has to be enforced at the point of booking rather than counted at the door.
Credits expire and people forget. A student with three classes left and two weeks to use them should be told, because the alternative is somebody who feels they wasted money and quietly stops coming.
Cover is a weekly problem. A teacher unavailable on Thursday is not a cancelled class if there is a list of who else can teach that style, and the students who arrive to a locked door do not come back.
Attendance is the early warning. Somebody who came twice a week and has not been for a fortnight has not got busy, they have stopped, and a message at that point is what brings them back.