Most public school systems assign pupils by home address. The map that performs that assignment determines school composition more directly than any stated admissions policy.
A map is an allocation mechanism
An attendance boundary converts residential geography into school rolls. Where housing is segregated by income or background, the roll inherits that pattern automatically.
The same set of schools can be given very different intakes by redrawing lines, because small changes at the edges move whole streets between catchments.
This is why boundary reviews attract intense attention. The decision looks technical but reallocates access to schools that families have chosen homes to reach.
Capacity drives most redrawing
Boundaries are usually revisited because one school is overcrowded and another has space. The stated purpose is balancing numbers rather than composition.
Planners work from projections of births, housing developments and migration, all of which carry uncertainty. A projection that proves wrong forces another review a few years later.
Because capacity is the driver, composition effects arrive as by-products of decisions taken for other reasons, and they are not always modelled in advance.
Housing prices respond to the lines
Where a school is regarded as desirable, homes inside its catchment command a premium. The boundary is capitalised into property values within a short period of being announced.
That premium acts as an admissions price. Families able to pay it gain access; families who cannot are allocated elsewhere despite formally equal rules.
The relationship is self-reinforcing, since intake affects reputation and reputation affects prices, tightening the link between housing cost and school assignment.
Choice programmes sit on top of the map
Open enrolment, magnet schools and lotteries allow some pupils to attend outside their catchment, which loosens the geographic link without removing it.
Participation depends on information, application deadlines and transport. Families with less flexibility use these routes less, so choice can widen differences rather than narrow them.
Systems that provide transport and default applications see broader participation, which suggests the barrier is often practical rather than a matter of preference.
Composition affects more than peer groups
Intake influences staffing allocations, subject offerings and the resources parent bodies can raise, so composition and provision tend to move together.
Schools serving concentrated disadvantage often face higher pupil mobility, which disrupts teaching for everyone in the class regardless of individual circumstances.
Rules governing boundaries, choice and transport are set locally and revised regularly, so arrangements in one district describe little about the one next to it.