The mad scramble for results and the publication of league tables towards the end of the summer break demonstrate the way schools can be used as a means of sorting the sheep from the goats. Which is meant to be preferable I'm never quite sure - I've always assumed sheep - but I digress.
The main point is that schools, through the examination system, divide from both within and without. From within, because they differentiate one pupil from another; from without, because they mark out the successful school from the one that is struggling.
Results are not used only as indicators of academic achievement, however.
They become a code for other, bigger social issues that can divide us, such as class and, with the possible, and planned, increase of faith schools - religion. Yet, as a recent Radio 4 series on the advent of the comprehensive reminded us, schools can also be used as a place of social engineering to create a more inclusive society.
The original advocates of the comprehensive wanted to place children from all walks of life under one roof - and not, as is often claimed, because they believed one size fits all. Part of the argument, at least, was that only when you have grown up together, rather than being segregated, do you feel genuinely tied by a sense of common community. The comprehensive ideal has been much criticised in the last few years, but it is partly because it has had to function in a market system which is designed to undermine it.
Any market is, in the end, about winners and losers, about those who make it and those who are left behind. The comprehensive has always existed in an educational world that has maintained highly selective independent schools as well as the 11-plus in some local authorities, particularly in the South-east, where competition is acute.
But despite its very bad press and the difficult circumstances in which it has had to operate, far from being a dismal failure, the comprehensive has, in most cases, done its job. Most children in this country attend their local school and more than twice as many achieve the examination success of three decades ago when the comprehensive era dawned. In London, the area I know best, pupils who live in million-pound houses sit in class with children from the local estate and socialise together out of hours. Schools draw on an eclectic mix of ethnic minorities and often more than 60 different languages are represented within the school gates.
This is what the idealists of the 1960s had in mind when they began the comprehensive movement, and it seems a vision worth fighting for because there is still much room for improvement. The haunting images of the hurricane victims in New Orleans showed starkly what a divided society can look like.
Social engineering works both ways. Designing a marketplace in which only a few gain is as much about creating the kind of society we envisage for the future as one that attempts to combat the divisiveness of inherited privilege and poverty. I know which future I'd rather inhabit.