Choice and diversity in the secondary school system mean that parents can choose a school which meets the particular needs of their child. Choice also means that good schools will thrive and bad ones will die. At least, that's the theory.
It is invariably discussed in the context of secondary transfer at age 11.
But how does it relate to the situation of children who seek a school place after the normal age of transfer from primary school? What choice do they have?
Every year, thousands of children enter secondary schools at non-standard times. Some have parents who are moving home at the behest of their employers or to advance their careers - for example, armed forces personnel, bank managers and clergy. Much of the residential movement affecting secondary schools occurs in city areas and involves families who are poor or dispossessed.
They include the following: foreign workers coming to the UK to do low-paid jobs; refugees and asylum-seekers; families who have become homeless through poverty, debt and eviction; families fleeing domestic or neighbourhood violence; parts of families who have split up; and families who move frequently as a way of life.
The children in these families are joined in the quest for school places by others "on the move", such as children moving between foster parents or separated parents; those excluded from their previous school or leaving to pre-empt exclusion; and those going to live with Granny because parents have gone to prison or cannot cope for some other reason.
Some of the above fall into the groups which the Government's five-year strategy for children and learners had in mind when it asserted that: "we fail our most disadvantaged children - those in public care, those with complex family lives, and those most at risk of drug abuse, teenage pregnancy and involvement in criminal activity."
At the same time, many have the potential to achieve a great deal if they are given the help they need to overcome barriers to achievement.
These children do not have a great deal of choice when it comes to secondary schools. They are most likely to get into the ones which were least popular with parents at secondary transfer and so have many spare places. Thus, schools which parental choice or "the market" have consigned to a slow death take on the most difficult and arguably the most important task of all: to support and educate some of the most disadvantaged children in society. Moreover, their intake of 11-year-olds at the start of Year 7 will invariably have included many such children.
Some of these schools cope astonishingly well with the pressures they are under and sustain a positive and welcoming atmosphere. They have exceptional headteachers and senior staff who keep the focus on the needs of their particular pupils and get their rewards from the progress and achievement that result - even if it does not take the form of 90 per cent with five or more A* to C grades at GCSE.
Often, however, such schools get into serious difficulties. Heads have breakdowns. Staff come and go. Pupil behaviour deteriorates and so does teaching and learning. In the end, closure may be seen as the only option.
Ironically, when this happens, the residual pupils tend to move on to other schools under similar stress.
Some of the new academies are being created from schools which have hitherto been undersubscribed and had high levels of pupil mobility. If the investment, publicity and effort that goes into establishing them result in greater popularity, it is likely over time that they will fill all their places at secondary transfer, drawing mainly from the more settled parts of local communities. If this happens, their reception function - that is to say, their role in admitting children new to the area or moving in and out of temporary homes - will pass on to other institutions. Before long, another local school may be struggling.
This is not an argument against strategies to strengthen schools in deprived areas. It is an argument against the expectation that "choice and diversity" will result in a levelling up of schools, with every school having the capacity to develop the potential of all its pupils.
The Government is not unaware of the pressure that some schools are under.
The recent pronouncement by Education Secretary Charles Clarke that all schools are to be required to share responsibility for excluded pupils suggests a growing realisation of struggling schools' disproportionate burden at present.
However, other policies such as the encouragement of foundation status, with more schools becoming their own admissions authority, may actually make it more difficult to take account of mobility in a co-ordinated way.
There is no simple way to create and maintain a system in which all schools are "good" schools. But perhaps we would get closer to creating one if we stopped relying on market theory and looked at some of the other forces which shape our school communities.
Dr Janet Dobson is a senior research fellow in the migration research unit at University college, London. She is due to speak at a conference, Pupil Mobility: implications for schools and LEAs in London today, organised by The Education Network. www.ten.info. A new research report by Janet Dobson and Claire Pooley on pupil mobility in secondary schools can be obtained from firstname.lastname@example.org