Traveller children are "disappearing" from schools because of the pressure on heads to raise results, an experienced local authority adviser has said.
Lucy Beckett, head of Oxfordshire's traveller education service, said some schools resisted taking on travellers because of the children's reputation for poor grades and erratic attendance.
The traveller children could then sometimes drop out of education altogether as a result.
Exam scores and truancy are key measures by which schools' effectiveness is gauged, primarily through league tables.
Travellers - classed as "GypsyRomany" or "traveller of Irish heritage" children in the Government's results statistics - have the lowest exam achievement of any ethnic group.
Only 14 per cent of GypsyRomany pupils achieved five good GCSEs in 2007, the latest official data show, with travellers of Irish heritage on only 16 per cent.
Drop-out rates as schooling progresses are also high: numbers of GypsyRomany pupils taking GCSE or equivalent exams are less than a third of those at key stage 1, while for travellers of Irish heritage the corresponding figure is about half.
"For those of us who have worked in traveller education for a long time, there are concerns," Ms Beckett told a fringe meeting of a Runnymede Trust conference on race equality.
"Last week, I had a headteacher say directly to me, 'Please can you find a way of taking all the traveller children out of my school so they can be taught separately.' This is still being said.
"A lot of this is about the burden of procedures and expectations in terms of bureaucracy and in terms of published results.
"Schools are under a huge amount of pressure to look good in terms of their published results, their attendance figures and so on.
"It's very important there is still a focus on these children to make sure that they do matter, because there is a danger that, given the overwhelming need to be demonstrating success and achievement, some of the children who may already have been excluded and who do not fit with the targets may simply disappear."
One conference participant said that, given the pressures on them, some schools were reluctant to take on travellers.
Ms Beckett said: "Schools say, 'This is going to be awful for us. The (pupil's) Sats results are going to be awful, and the attendance is going to be awful.' They cannot see beyond the figures."
Last year, The TES reported how campaigners for children in care and those with special needs believed league tables were pushing some schools to reject such pupils because they felt they were unlikely to help the rankings.
Sarah Lewis, a Department for Children, Schools and Families official, told the Runnymede meeting that the Government was conscious of the pressure on schools to raise results. Ofsted was trialling the introduction of a new league table indicator, focusing on measuring pupil wellbeing, designed to be an alternative to pupils' results as a measure of success.
Ms Lewis said: "Schools are under a lot of pressure, and we have to be quite supportive of that. It's about educating the whole child."