Special schools and pupil referral units risk breaking the law because they will find it impossible to comply with the new "rarely cover" regulations coming into force this term, headteachers have warned.
Heads' leaders are seeking an urgent meeting with Schools Secretary Ed Balls to discuss the changes under which teachers should not be routinely expected to fill in for absent colleagues.
SEN staff and those working in PRUs say they have little choice but to cover lessons themselves due to a severe shortage of specialist supply teachers and because bringing in outsiders upsets children.
Changes this year mean teachers in special schools face being visited by Ofsted more often their mainstream colleagues, and they fear they will be the first to face the wrath of inspectors if they cannot comply with the legislation. Rarely cover is the last important piece of the national workload agreement to be brought in.
National Association of Head Teachers representatives say they have repeatedly warned the DCSF of the need to give special schools and PRUs more flexibility.
In a letter to Mr Balls, NAHT general secretary Mick Brookes said: "We have been contacted by a considerable number of members who feel strongly that there is little appreciation of the impact rarely cover will have, particularly in settings which cater for pupils on the autistic spectrum and those with socialemotionalbehavioural difficulties.
"These pupils require continuity of teaching staff, differentiated teaching and pupilteacher interaction to maintain quality provision. The availability of appropriate short-term supply cover is limited by the very nature and needs of the children. Inappropriate short-term strategies undermine standards and increase workload because of the detrimental effect on the behaviour and stability of pupils."
The other unions are not reporting the same level of discontent among mainstream headteachers, who either trialed the change last term or have already introduced it.
"It will be a challenge for schools, especially those who rely on teachers to cover. Headteachers will have to rethink absence policies," said John Dunsford, Association of School and College Leaders general secretary.
"Secondary schools will find it much harder."
But NASUWT general secretary Chris Keates doubted whether rarely cover was posing many problems in mainstream schools.
"NASUWT has spoken to many of our school leader and teacher members in schools and they have not expressed any concerns. Indeed some have been operating rarely cover for a long time," he said.
The Department for Children, Schools and Families refused to comment.
Rarely cover means that teachers and headteachers can be asked to cover lessons only rarely and only in circumstances that are not foreseeable.
The School Teachers' Pay and Conditions Document says cover for an absent colleague is not a good use of the time of a teacher or a headteacher.
It will not be acceptable for heads to say some trips or some continuing professional development activities can only take place if teachers provide cover.
There is no requirement on schools to employ cover supervisors. They could employ supply teachers, support staff, teachers specifically for cover or use agency staff.
Cover supervisors and higher-level teaching assistants are for short-term absences only.
Medium and long-term absences should be covered by a teacher.