'White flight'

Some Christchurch schools are less than two-thirds full – and one is languishing at less than a third capacity – as the city's popular schools expand at their expense .... parents are deserting the eastern suburbs for schools with whiter, richer students.
This is probably quite a general phenomenon.
Unfortunately, fewer students means: fewer resources and fewer teachers - to maintain numbers they tend to enrol those kids that their counterparts don't want or have actively rejected. So more 'difficult to manage, difficult to teach' kids, teachers of lesser quality - coz who really wants to work under such circumstances - increasing focus on social behaviour which begats a greater need to focus on social behaviour and a 'necessarily' lesser focus on teaching and hence less learning.
Everyone knows, bar a lot of school teachers, that greater structure, formality, learning to fluency, are important in kids effectively remaining motivated and doing well at school ... and after all the reason for compulsory attendance is to 'acquire an education.'
'White flight' has connotations of 'running from something not white' that is aversive and has racist overtones.
'Good parenting' demands you place your kids where they can maximise their opportunity to do well educationally .... and this process will, naturally result in the 30% of non performers or failures increasingly residing under the one roof - zoning has not and realistically should not stop this occurring ... because really this is all indicative of a failing education system.