In the past century, she explained, all the inputs into secondary schooling were held constant. We taught all young people in the same way, in the same room, for the same length of time and the result was wholly predictable - their academic performance varied hugely. Failure was attributed not to this approach to schooling but to the young person’s background or “brightness”. In scientific terms, the inputs were constant and standards were the variable.

Now, in stark contrast, she points out, Colorado asserts that all students can achieve. In other words, high standards have become the constant. The consequence is, she argues, that the inputs must become variables. In addition to proven whole-class methodologies, teachers need to adapt their teaching to meet individual needs and aspirations if all students are to make progress.

They need to find the different keys to motivating different teenagers. When a young person isn’t achieving, Lois Easton argues, teachers can no longer blame background factors or innate ability. Instead, they need to ask: “How can I teach differently to enable these students to achieve high standards?”

But it’s not only pedagogy that needs to become a variable. What about time? If a young person is falling behind his or her peers, why not provide extra tuition after school, at the weekend or in the holidays? What about place? If a young person is uninspired in a traditional classroom, why not enable them to learn some of the time, in the workplace, the community or in front of a computer at home? Why not try some outdoor adventure? And tying all these together, what about resources? The cost of enabling some young people to achieve wll, given their starting points, be much greater than for others.

Therefore, if we want all children to achieve high standards. Resources, over and above the entitlement for all, must become a variable too.

Lois Easton’s simple but powerful slogan “If standards are the constant, everything else must become a variable” seems to summarise the direction of a central thrust of government policy in this country. Of course there’s a long way to go but consider some examples.

By providing opportunities for out-of-school learning, booster classes and summer schools, the Government has been moving towards the notion of time as a variable, not a constant. By opening up the opportunity at key stage 4 for pupils to pursue a wider range of qualifications and to engage in work-based learning the Government is enabling content and place to become the variables.

No metaphor is perfect of course. While we do want standards as a constant, in the sense of every pupil achieving well in the essential core of learning (literacy, numeracy, learning how to learn), we want some to go far beyond. And in areas outside the core we want pupils to pursue their aspirations as far as they can go. To take art as an example, we surely want every young person to be able to draw and understand perspective but above and beyond that we want to see work of dazzling diversity. Similarly in reading and writing we want every student to be highly capable but we also clearly want them to read diverse texts and write in diverse styles about diverse subjects.

Nevertheless, Lois Easton’s metaphor is very helpful. It’s a different way of expressing Education Secretary David Blunkett’s oft-repeated sentiment that “poverty is no excuse for underperformance, but it is a reason for targeted support”.

Michael Barber heads the Government’s standards and effectiveness unit