Then it's clearly evident that you should support this "mostly white city" because the benefit they'd gain by not being chained to the poor urban schools would almost certainly be way, way above 10%.
That bullshit is writ large all over the country with the haves seeking even greater advantage over the have-nots at every opportunity. It ends up being defined in terms of race rather than class which just reinforces that tendency, sad to say.
Lots of kids of all races attend lousy schools so that kids further up the economic food chain can have better. Affluent parents are fine with that just so long as their kids get good schools & will remain so if they can separate themselves from the larger needs of the community & the nation in just the manner you advocate.
A poorly educated citizenry is a liability to the nation. Don't believe me? Witness Trump as President.