Labour has revealed plans to tackle the problem of “off-rolling” in schools, when headteachers exclude students who are likely to do badly in their GCSEs because their results could compromise the school’s performance in league tables.

Recent analysis by the schools watchdog, Ofsted, found that more than 19,000 pupils who were in year 10 of secondary school in 2016 vanished from the school roll by the start of year 11, when they sit GCSEs, of which around half do not reappear in any state school.

Now Labour wants to remove the incentive for heads to off-roll pupils by making schools accountable for the exam results of students who leave until they have another permanent school place. This would in turn encourage schools to help the excluded pupil to find the best possible alternative provision.

In an interview with the Guardian before next week’s Labour party conference, shadow education secretary Angela Rayner raised her concerns about off-rolling. “Teachers and heads want to do the best for all their pupils, but too often they find themselves caught in a system that doesn’t reward them for doing the right thing.

“As it is, some of the most vulnerable are being let down or falling between the gaps of an increasingly fragmented system and out of education altogether. There needs to be a clear responsibility for these children, and accountability for their educational outcomes.”

Off-rolling disproportionately affects vulnerable and disadvantaged pupils. Around 30% of those who leave their school between years 10 and 11 have special educational needs; pupils eligible for free school meals, children looked after by local authorities and some minority ethnic groups are also more likely be affected.

The problem is worse in London, where a higher proportion of schools are losing pupils compared with other areas of the country and academies are losing proportionately more pupils than local authority schools.

Labour would also like to close a loophole that allows schools to receive funding for a pupil before they are counted towards exam results. According to Labour this creates a window in which pupils can be off-rolled without consequences, having already generated additional revenue for the school.

Rayner, who is due to give a keynote conference speech in Liverpool next Monday, said: “We must all take responsibility for our must vulnerable children, and that is why we will reform the accountability system to support them, while moving towards a system that is cooperative, gets teacher workload under control, and gives schools the resources they need.

“That is why Labour will close the loopholes and end the perverse incentives. This will ensure that there is no unintended reward for so-called off-rolling and give schools an investment in ensuring that excluded pupils move on to the best possible provision.”

Philip Nye, a researcher with FFT Education Datalab, said: “Our own research has shown the need for action over off-rolling, so it’s positive to see Labour putting forward this proposal. While a step in the right direction, we’re concerned that under this proposal, pupils could still be encouraged out and on to the roll of another school operating below capacity.”