Disruptive children who have special needs are treated more leniently by teachers than their peers if they behave badly, according to a report.
The study, from the National Foundation for Educational Research, also found no evidence that those with special needs are discriminated against in admissions procedures.
Admissions and Exclusions of Pupils with Special Educational Needs was carried out amid "unease, illustrated... in anecdotal evidence, that some vulnerable children were disadvantaged by schools' policies".
It questioned teachers and other professionals, parents and pupils attending 17 primary, secondary and special schools in six local education authorities, and found that, although children with special needs tended to be treated similarly to other pupils, "thresholds were often higher and a greater degree of unacceptable behaviour was tolerated before the exclusion... was initiated".
The report added: "Schools were particularly reluctant to exclude pupils in cases where they felt adequate alternative provision was not available."
It found no evidence of systematic discrimination or unfavourable treatment of children with special needs when it came to school admissions.
"Schools did not have an opportunity to do this as information about pupils' needs was not available when the admissions criteria were being applied," the study said.
Schools tended to respect the legal position regarding admitting pupils for whom the school was named on the statement. However, there was sometimes less favourable treatment when pupils had no statement, the study found.
Schools often tried to dissuade parents from seeking a place for their child, though this was more difficult if the school was not full.
The study said: "Reluctance to admit a pupil with SEN usually occurred when schools considered that they already had a high proportion of pupils with SEN andor they could not cope with a particular profile of needs, and that they had inadequate financial resources for meeting the needs of the pupil."
Casual admissions during the academic year also caused difficulties because schools were unable to make plans, and often did not have the appropriate provision in place.
The report said local authorities should ensure a range of provision was available to suit all children, and exclusions should be monitored and challenged if inappropriate.
It recommended that schools should make teachers aware of whether a child's poor behaviour was as a result of special needs, or because the pupil was demotivated. They should also make sure staff react appropriately to pupil behaviour and "help pupils acquire desirable patterns of behaviour".
Schools should also be encouraged to consider the effectiveness of exclusion and to question whether other sanctions could be used.
However, support organisations for pupils with special needs were sceptical about some of the findings.
"Official figures show that two-thirds of excluded pupils have some form of special needs, so it is simply not true that these children are treated more leniently," said John Wright, spokesperson for the Independent Panel for Special Education Advice. He added that schools were only fulfilling their legal requirements when admitting children with statements.
Chris Darlington, past president of the National Association for Special Education Needs, said: "Schools are accepting that exclusion is no longer an option and they know there are expectations on them when it comes to admissions...
"(They) have an understanding of the philosophy and principles of exclusion; however there remains a divide between that practice and meeting the needs of all youngsters."
The report can be seen at www.dfes.gov.ukresearch