TENS of thousands of pupils may have been informally excluded by schools desperate to keep their positions in performance tables, even though the practice is illegal.

The results of more than 30,000 pupils have not been reported in league tables despite the children appearing on school registers for the previous three years, an investigation by the Times found.

There were 539,844 Year 10 pupils at state schools in 2016, but the results of only 526,956 could be found a year later – a drop of 12,888.

While pupils moving abroad or to non-state education accounts for a proportion of these figures, the numbers of pupils expelled in the months before the exams of the last two years were 9,237 and 9,136.

This means that 31,261 pupils have not been included in official statistics.

A Department for Education spokeswoman said informal or unofficial exclusions are unlawful.

She added: “We wrote to schools last year to remind them of the rules on exclusions. Any school ‘off-rolling’ on the basis of academic results is quite simply breaking the law.”

Schools watchdog Ofsted expressed its concern about the off-rolling practice, saying that using exclusion to boost school performance is “never acceptable.”

Ofsted deputy chief inspector Matthew Coffey said: “We are increasingly concerned about the scale of off-rolling apparent in some secondary schools.

“While we support the right of schools to exclude individuals who disrupt other pupils’ ability to learn, particularly those who exhibit violent or threatening behaviour, it is never acceptable to use exclusion to boost school performance.”

The analysis comes after research published in June claimed that last year some 22,000 children left state secondary schools before the end of Year 11, which was more than in the previous three years combined.

Concerns have been consistently voiced by trade union activists, who are concerned that the current way of ranking the educational qualities of state schools encourages counterproductive “zero-tolerance” policies on bad behaviour.

National Education Union joint general secretary Mary Bousted recently called off-rolling a “perverse consequence” of the current system, which makes schools feel “penalised” for “teaching pupils with additional needs.

“To best counter this, the government should change the accountability system so schools are given credit for working with children with complex needs,” she added.