At the annual conference of the Association of School and College Leaders last week, we launched a new report on school funding: The True Cost of Education. The report was an attempt to set out the education that ASCL believes children and young people are entitled to in England in the 21st century, and what that costs.

The report’s headline finding, that schools need £5.7 billion more in 2019-20 to deliver that education than is currently provided in the government’s budget, attracted much attention. Commentators speculated on what game ASCL was playing. Have we scuppered our chances of persuading the Treasury to part with more cash for schools by asking for an impossible sum? Or were we deliberately pitching high, in the hope of compromising on a lower amount?

The truth is, we’re playing this with a straight bat. We’re not politicians, and we’re not economists. What we are is an organisation that works day in, day out, with our 19,000 members to help them to lead their schools and colleges as effectively, and efficiently, as they possibly can. Using the government’s own preferred methodology, known as Integrated Curriculum Financial Planning (ICFP), we have a team of experts who work with schools and colleges to consider the education they want to provide to the children and young people they serve, and how they can afford to do that.

The True Cost of Education takes that methodology, and scales it up to a national level. It’s built around seven assumptions, which we think form the absolute minimum that children growing up in one of the richest nations in the world should be entitled to expect. These include that children should be taught by a qualified teacher 100 per cent of the time, that they should be taught in classes of no more than 30 pupils, and that they should be taught by teachers who are given adequate time to prepare lessons and mark children’s work.

The cost of quality education

These expectations are not, we believe, unreasonable. We could have pushed for more. We could, for example, have attempted to model in more detail the cost of delivering the type of broad, rich, deep curriculum that Ofsted is advocating in its draft revised inspection framework, and which school leaders are desperate to provide. And we could have said much more - and may in future - about the additional costs involved in supporting children and young people with increasingly complex needs, and from families struggling to cope with the impact of years of austerity.

We chose, though, to keep things simple. We chose to focus on the basics, and to come up with a figure which we think genuinely reflects the amount of money our schools need to deliver that basic education.

The assumptions we’ve made, and the methodology we used to reach the figure we came to, are all included in the report. We wanted to be as clear and transparent about this as possible, in order to start an open, honest conversation. We’re more than happy to be challenged both on our underlying assumptions and on the figures we used to reach our final sum.

The True Cost of Education was originally inspired by a question we were asked by Robert Halfon, chair of the Commons Education Select committee, during its recent inquiry into school funding. How much money, he asked, do schools actually need?

This report is our attempt to answer that question. The answer may not be popular. We believe it reflects, though, the true cost of providing the basic education to which our children are entitled. Our children’s futures are too important to play games with.

Julie McCulloch is the director of policy at the Association of School and College Leaders