The question itself falls apart when you remove the requirement that all schools follow a rigid curriculum based on grade levels modeled upon a particular age cohort.
If you accept that a 9-year-old student can learn productively in the same room as an 11-year-old student, you no longer need to consider setting aside a separate room for all the 9-year-olds with ability equal to the median 11-year-old. You just put the smart kids into the more advanced classes.
Just let the fast kids read ahead on their own. If they can get far enough ahead of the instructor, they can skip a semester, and either start the next class in the sequence early, or cram an elective class into the gap. There's no need to ship them off to another building.
If you accept that a 9-year-old student can learn productively in the same room as an 11-year-old student, you no longer need to consider setting aside a separate room for all the 9-year-olds with ability equal to the median 11-year-old. You just put the smart kids into the more advanced classes.
Just let the fast kids read ahead on their own. If they can get far enough ahead of the instructor, they can skip a semester, and either start the next class in the sequence early, or cram an elective class into the gap. There's no need to ship them off to another building.