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But if there is only one level of offering, either the best students will seek resources elsewhere, or the slowest students are left behind. Parents on this forum would be very frustrated if their kids are in a school where all kids have to learn math at the same pace. But this is the same if all kids have to be in the same orchestra learning the same pieces.

No objection here at all to multiple levels. I think that's great. The district in question has multiple levels, and performance requirements were in place for the lowest level.