Where did all the babies go?
You have to blame the parents. They're just not doing their bit.
The fact is, we're failing to keep the maternity wards busy the way we used to. The birth rate has been sliding since 1990 and the effects are now filtering through to the primary school population: down 38,000 last year and 56,000 this. And it's going to get worse.
The suspicion is that the Department for Education and Skills has been slow to respond. This week it refused even to issue projections for pupil numbers and appeared to shrug its shoulders of any responsibility.
"Decisions about reductions in the size of schools, amalgamations of schools or the closure of schools - are primarily for governing bodies, local education authorities and school organisation committees," officials said in a statement when asked if they had a long-term strategy. The Welsh Assembly takes a similar view.
Nevertheless, it appears to be dawning on ministers that there is a problem. Last month's funding plans offered transitional protection for schools losing pupils - for two years at least.
And, while the decision to bring in accountants KPMG to advise heads on balancing the books had a patronising tinge, it did at least acknowledge that the challenge for school leaders is increasing.
For all its coyness, the DfES provided projections in evidence to the School Teachers' Review Body in July. It predicts that more than 600,000 students will be lost over 15 years from the peak of 2001. Primary schools will go on losing 50,000-60,000 pupils a year until 2006, and shrink till 2012. Over the decade, they are expected to lose 450,000 pupils - just over 10 per cent.
That will start to hit secondary schools in 2005. By 2016, they will have lost 350,000 students or almost 11 per cent - one in nine.
This is, of course, part of the natural cycle of baby-boom-and-bust. Nor are the figures as dramatic as the last trough in the cycle, the 1980s and early 1990s when numbers fell by a quarter. Then, primaries lost 1.2 million pupils, while secondaries lost 1m. In three successive years, more than 200,000 children disappeared from the system.
But this latest fall is still going to have a huge impact: the figures equate to losing some 20,000 primary teachers and almost 21,000 secondary.
"We've already had problems with funding, so falling rolls really is a double whammy," says Jane Phillips, chair of the National Association of Governors and Managers. "The long-term problem is that some schools will have to close and that is always difficult and emotional."
If it is left to councils and schools, managing change won't be easy. In a joint report School Place Planning the Office of Standards in Education and the Audit Commission said planning places was an exercise of "enormous complexity" that aroused strong passions in parents.
Local authorities are in a much trickier position than they were in the 1980s trough. Since then, huge amounts of autonomy have been handed to schools. Heads and governors now run their own budgets, foundation schools set their own admissions. So, unlike the 1980s, LEAs cannot easily redeploy redundant staff.
Still, many have grasped the nettle. One reason falling rolls may have less impact this time is that many LEAs have squeezed the fat out of the system.
The past 10 years have seen school closures, mergers of infant and junior schools and the disappearance of middle schools.
The pressure for this began with the previous Tory government, which ordered LEAs to remove surplus places. The number of spare secondary places fell and overcrowding rose. (former Tory education spokesman Damian Green dumped this old policy, and said falling rolls should be used to create surplus places needed for parents to exercise more choice.) At both primary and secondary level, even though rolls climbed for most of the 1990s, the number of schools actually fell by around 1,300. Indeed, they have been falling since 1983.
What will the effect be on teacher training? Last time, dozens of colleges closed or merged. But this time, the DfES is saying there are no plans to cut recruitment. With a quarter of the profession in their 50s, retirements will fuel demand for new blood.
Unions say falling rolls are the perfect opportunity to make the workload agreement work. If staffing levels stay the same while pupil numbers go down, it will give teachers the non-contact time they're promised.
That would require a change to pupil-led funding to allow shrinking schools to keep their budgets. Education analyst Alan Smithers of Liverpool university suggests primaries should get more funding on a per-school basis so they are less vulnerable to swings in pupil numbers.
But Education Secretary Charles Clarke last week seemed to rule that out, suggesting redundancies are inevitable. Yet the DfES says it has no intention of using falling rolls as an excuse to cut education spending. so where will the money saved go?
Perhaps it will be used to implement radical ideas that have been floating around for a while. We could see primaries turn themselves into extended or "full service" schools, incorporating housing, social and health services, youth clubs, adult education and more.
Post-11 cash could be channelled into overhauling school buildings.
In Portsmouth, so tight for space in the 1990s it built two new secondary schools, pupils equivalent to one school will be lost. But the city has still bid to be in the first wave of the rebuilding programme.
DiAnne Smith, head of Admiral Lord Nelson school, envisages buildings that allow a new type of learning. "We're not going to have 25 kids with one teacher for 30 periods a week." With personalised learning and more vocational courses for 14 to 19-year-olds, some will be in smaller tutor groups, some will be off-site, some will be in lecture theatres and some will be in ICT suites.
Small rural primaries look likely to come under scrutiny again. Politicians may balk at closing village primaries, but the National Association of Head Teachers predicts a growth in federations of small schools often under a single head.
And the question of what to do with empty schools will return. Some might be converted (the Lib Dems have suggested the creation of high-quality early years centres) but the truth is, many schools that close are in too poor a condition to do this. Most LEAs would rather build from scratch once the numbers warrant it.
Private schools don't forecast many problems, provided the economy stays buoyant. Dick Davison, spokesman for the Independent Schools Council, says numbers have risen every year since 1983 bar the Major recession of the early 1990s.
New powers under the 2002 Education Act and the new admissions code of practice should put LEAs in a better position to manage falling rolls. But the job remains hugely complex. Some LEAs have to liaise with dozens of admissions authorities, from foundation schools to church dioceses. Ofsted has already asked whether a body larger than local authorities is needed to manage places, particularly in London.
We'll probably never again see an annual birth rate of 800,000-plus as we did in the 1960s. The last peak in 1990 was barely 700,000, and the trend towards later and smaller families means the pattern is changing.
But if the British public wants to prevent problems ahead, they know where their duty lies. It really is a case of lie back and think of England.