I don't think the concern would be that "they could take over the whole society". It would be more that smart people (more accurately: people in various groups that correlate with smartness, and perhaps more strongly with schools' estimates of pupil-smartness) already have some tendency to interact only with one another, and segregating schools would increase that tendency, and that might be bad for social cohesion and even stability (because e.g. those smart people will include most of the politicians, and the less aware they are of what Ordinary People want the more likely they are to seem out of touch and lead to populist smash-everything moves).
that might be bad for social cohesion and even stability
This is a complicated argument. Are you basically saying that it's "good" (we'll leave aside figuring out what it means for a second) for people to be tribal at the nation-state level but bad for them to be tribal at more granular levels?
For most cohesion you want a very homogeneous population (see e.g. Iceland). Technically speaking, any diversity reduces social cohesion and diversity in IQ is just one example of that. If you're worried about cohesion and stability, any diversity is &quo...
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