The attainment gap between disadvantaged pupils and their peers is the “big civil rights challenge” facing schools, a senior government official has said.

Sir David Carter, the national schools commissioner, warned of a “stark” difference in achievement between better-off pupils and their poorer peers, who were shown to be on average 18 months behind in last year’s GCSE results.

I think this is our civil rights challenge in this country, to close this gap

Carter, whose team of commissioners is responsible for taking action in underperforming schools, said leaders had to recognise the scale of the attainment gap.

“I think this is our civil rights challenge in this country, to close this gap,” he said. “Our schools don’t belong to us. Our schools belong the communities we serve. Our job in the social mobility space is to make sure that when we hand that baton over, we hand it over in a better position than it was handed to us.”

He wants schools to recognise their role as community leaders “as community leaders as well as educational leaders”, as they have a duty to build relationships with the public and charitable sectors and “shine the light on disadvantage”.

He also warned of the limitations of school accountability measures, and said social mobility “can’t be measured by a set of Progress 8 scores and GCSEs”.

It would be more effective to measure the performance of young people when they reach their 25th birthday, Carter said.

“The true test of your leadership, and your teachers’ quality as pedagogists, is what happens to those children afterwards. I think that’s one of our greatest challenges when looking at vulnerable communities.”

Carter commended the work of multi-academy trusts in improving social mobility, saying academies had “historic success” in turning around schools in some of the most deprived areas of the country.

He called for more collaboration between academy trusts and schools, and urged chains to “over-recruit” teachers and leaders so they could help turn around schools outside their organisations.

“MATs who are recruiting just enough teachers for their organisation will not be able to play that wider system role,” he said.