How do you square parental choice with trying to ensure that all schools have a balanced, comprehensive intake? This question, which has exercised the minds of the great and good in education since comprehensives were introduced more than 30 years ago, has been thrown into sharp focus by the Government's increased emphasis on competition and specialisation.
Even Labour, the party, most closely associated with comprehensives, has made it clear it does not expect its commitment to balanced intakes to override the right of parents to choose the best for their children.
This commitment was dramatically reinforced recently by the decision by health spokeswoman Harriet Harman to choose a selective grant-maintained school for her son rather than a local comprehensive.
The intense debate now opening up about the future of comprehensive education is now being taken up in four inner London boroughs - the last in the country to use a banding system to determine which secondary school children will attend.
The boroughs - Lewisham, Hackney, Greenwich and Tower Hamlets - argue that it means schools have a spread of ability in comprehensive schools.
But parents have attacked it as unfair because it means bright children are sometimes denied their choice of school even though there are vacancies.
Before it was abolished in 1990, the Inner London Education Authority operated a unique system of banding which took priority over all other admissions criteria including siblings, distance from school and medical or social need.
Children in the 12 inner London boroughs took the London Reading Test which was used to group them into three ability bands. Secondary schools were then given a quota of places for each band and only allowed to recruit pupils up to that limit, thus ensuring each school got its fair share of above and below-average pupils.
When the London boroughs took over the education service, however, an early decision to end banding was made by Conservative-controlled Kensington and Chelsea, Wandsworth and Westminster. Banding was then widely abandoned by Labour authorities too, including Hammersmith and Fulham, Islington, and Camden in 1992. Southwark and Lambeth soon followed suit.
Some grant-maintained schools now select by ability and a few maintained schools have sought to re-introduce a form of banding to ensure they get their fair share of bright children.
Lewisham believes its banding system ensures all children have an equal chance of getting in to their preferred school.
A recent report seems to support the position. Changing schools: secondary schools' admission policies in inner London in 1995, by Hazel Pennell and Anne West of the Centre for Educational Research, London School of Economics, showed that where banding had been dropped, a larger number of lower ability children were having trouble getting places.
A spokesman for Lewisham said: "Banding ensures we get a proper spread of ability throughout the borough. A lot of parents with children in high-ability bands target certain schools because they know they are good whereas parents with children in bands 2 and 3 do not bother to try.
"If we allowed all the high-ability pupils to go to the same schools the others will fail and an awful lot of kids will lose out."
Greenwich spokesman Andrew Stern added: "A good comprehensive mix has been shown to improve overall levels of attainment. If we did not share out the lower-ability pupils we would be in danger of creating a two-tier system. "
But Labour is cool about the banding system because of the difficulty of reconciling it with parental preference. A Labour adviser said: "Any planning of this kind would be seized upon by the Right as old Labour, socialist-type engineering. Regardless of its educational merits, it is unlikely to make a come-back."
Instead, Labour plans to ask all schools to reach agreement with their local authorities about their admissions policies, and there would be an independent system of national arbitration to handle disputes.
The party has also pledged to set up an independent tribunal to handle parental appeals which, it believes, will make the process fairer.
Sir Peter Newsam, a former chief education officer for ILEA, regrets the passing of what he regards as one of the best admissions systems.
But he believes that although banding worked quite well in a different age, it would be impossible to re-introduce now. "Parents would say they don't want people making judgments about their children in that way," he said.
Professor Peter Mortimore, a former director of research and statistics for ILEA and now director of the Institute of Education, London, said: "Banding offers a way forward but can only work if parents accept that to have the most number of schools that are going concerns they can't always have their first choice of school."