For various reasons, the school-age population of a school district in any given country is rarely stable. Population in some places grows unexpectedly, while in others it decreases, resulting in empty schools and wasted facilities. And since certain schools are more desired by teachers, parents and students than others, the problem of fair allocation of educational resources is aggravated further.

In China, the vast scale of rural-urban migration, economic inequality among different school districts and the relatively de-centralized methods of funding education exacerbate these problems.

A mobile population creates several types of educational problems in China. As parents from the countryside move to urban areas, cities grow and rural population shrinks. Rural schools lose students and become more expensive (per pupil) to maintain and, teachers, who want to raise their families in cities, try to shift from schools in remote rural areas to those in urban areas.

City schools are often reluctant to accept children of migrant workers, because authorities think their education levels are far below that of urban students and admitting them could lead to under-funded and over-crowded classrooms. Also, urban parents fear that their wards' studies would suffer if they share over-crowded classrooms with migrant workers' children.

During my travels in rural China over the past 25 years, I have met with rural teachers who were sent to the countryside in the 1970s and forced to spend the rest of their lives in rural schools, urban teachers who have not spent even a minute in a rural school, and teachers who used to be posted in rural schools but shifted to urban areas and thus created shortage of teachers in the countryside.

In one village school that I visited, the principal had to convince a 70-year-old retired teacher to resume teaching because no one else would take up the job. But because of his advancing years and failing health the teacher was often absent and the children's learning suffered.

In Australia, where I live, the population is mostly concentrated in a few large urban areas. In Western Australia, for example, there is one large city (Perth) and very many small towns in a vast, sparsely populated hinterland. Since few teachers want to settle permanently in the small towns, the state education department has made it mandatory for all teachers to teach for at least two years in a rural school before they can be considered for a permanent position in Perth.