Schools are excluding more children permanently for bad behaviour because of difficulties in arranging support for them, head teachers say.
Parents are responsible for children for an exclusion's first five days
The Association of School and College Leaders says rules to help vulnerable children stay in school are backfiring.
Pupils on temporary exclusions are the responsibility of head teachers after they have had five days out of school.
But the ASCL says a few days is not long enough to organise the mental health support such youngsters need.
The teaching union's survey of 50 schools found it took 80% of head teachers more than a month to access mental health support for children and teenagers.
About four out of 10 schools said it took more than three months.
ASCL president Brian Lightman told his union's conference in Brighton: "Earnest efforts to address the immense challenge of providing for the education of our most vulnerable young people are being undermined by the difficulty of accessing appropriate support services.
"This has begun to force schools to resort to permanent exclusions instead of longer fixed-term exclusions which might have enabled them to help these students.
He said five days was not long enough to organise referrals to mental health services or restorative justice sessions.
He added: "We are well aware of the detrimental effects of permanent exclusion and are trying hard to implement approaches which avoid the need to exclude.
"However we feel that the government's policy is backfiring and the pressure of stretched resources is getting worse as the year progresses."
Wander the streets
Under the rules brought in last September, parents of excluded children are responsible for the first five days and local authorities from day 15.
But schools are responsible for these pupils for the period in between.
This was introduced to ensure a child is not left to wander the streets during their exclusion and that their education continues.
A Department for Children, Schools and Families spokesman said he was not aware of any rise in the number of permanent exclusions.
"Permanent exclusion should always be a last resort - and in the case of children with special educational needs, who are sometimes the most vulnerable pupils, they should only be excluded in the most exceptional circumstances."
He added that the department was, however, aware that schools sometimes had difficulty accessing relevant services.