When an M.M. is specified for an exercise, say, quarter note equals 60 to 108, at what M.M. may the exercise be considered "mastered" so that the student may go on to the next exercise? How do piano teachers pace the students?

when I have this on music pieces or technique pages with students I have them play within the range given. For example, if it says 80 - 120 lets say (Im just pulling numbers out of the air as example) I would have them learn the piece at 80 and then have them speed up to 90 for the following week. If they seem comfortable there I have them try it out at 100 etc.. When they can't keep the timing properly due to speed and it sounds sloppy because of the beat then we go back to the highest count they were most comfortable with and perfect it at that speed.
I hope this makes sense, Im not sure if I explained it clearly - hard to think and type legibly at times...