Even if primary school has gone well, if they lose momentum now they have lost their chance for ever - and if primary school has gone badly, then this period is their last moment. Enter puberty barely able to read and write and worse, with no inbuilt habits about learning, and the game is up.
Britain's 4,000 or more comprehensives thus have an awesome responsibility. They are the custodians of the future lives of millions of British citizens. Often fashioned out of clumsy mergers and with catchment areas that largely predetermine their fate, they rank among the most pivotal of British institutions. Yet the sobriquet hung around their heads is "failure".
My own view about comprehensives has changed a lot over the past five years. British society is becoming more polarised. The rewards to the winners - both geographically and in terms of income - have exploded. Britain's economic hotspots, not just in the South-east, are in an increasingly virtuous upward spiral of success breeding success. But the cold spots are locked in no less a vicious downward spiral - and frequently the two districts are no more than a stone's throw apart. Think of parts of Leeds, Manchester, Glasgow or north London.
Inequality is now not merely about income - but about how you look and talk, where you live, the quality of the networks to which you belong, your facility with people and technology.
Comprehensives are one of the last lines of defence in binding the country together in near impossible circumstances - but that makes their task all the more challenging. Where comprehensives are successful there is a combination of educational and cultural vitality that is spellbinding; they have succeeded in harnessing youthful energy around a common cause and cultural egalitarianism hat is as refreshing as it is novel.
My judgment is that more succeed than fail - hence the vitality of British youth culture, the gradual but definite improvement in educational standards and the important new sense that the country is beginning to reinvent itself with the old icon of class much less important. The crisis in British conservatism is in part due to the new egalitarian attitudes bred by a generation of children educated in comprehensives.
But some are trapped in the cold spots, where the catchment area condemns them to low standards, classroom violence and minimal ambition - hardly helped by a demoralised and underpaid teaching profession.
Moreover, the cold spots which create these underprivileged catchment areas are increasing as inequality increases - and these are the comprehensives in crisis which give the entire secondary public educational system a bad name.
The Government's response is to break the comprehensive system up, boasting that by the end of the next parliament four in 10 secondary schools will be magnet or specialist schools of some type. Five years ago I would have applauded the move, but now I am less sure. The correct response should be to make the investment in specialisms within the comprehensive framework, and really to focus investment in the comprehensives that are in crisis.
There is even a case for investing in a new system of fifth and sixth-form colleges whose catchment areas could be wider than the boundaries that imprison many comprehensives - and which would reattract the children of middle-class families who opt out of the state system at 8 and 11.
If the Government's new efforts to persuade universities to recruit more state-school pupils are successful, the middle-class will want the imprimatur of state education to win their children Oxbridge opportunities. This will then be where comprehensives can compete. The policy must be to give them the wherewithal to do just that.
Will Hutton is chief executive of the Industrial Society, and author of 'The State We're In'