Tougher new targets introduced to curb the numbers of children persistently absent from lessons will discourage teachers from supporting pupils with serious illnesses, heads have warned.
The "arbitrary" changes will lead to schools being "unjustly labelled and blamed", according to the Association of School and College Leaders (ASCL).
Currently, pupils who miss 20 per cent - just over six weeks - of a school year are deemed persistently absent. From October, the Department for Education is changing this threshold to 15 per cent.
National figures showing the numbers of pupils who miss 12.5 per cent, 10 per cent and 5 per cent of lessons will also be published for the first time. This includes children on school rolls with long-term health problems and disabilities.
Ministers say the new threshold will ensure that schools take action sooner to deal with absence. They will look at the possibility of lowering it further over time.
But ASCL general secretary Brian Lightman said the new target will penalise teachers who are trying to support ill children.
"These young people may still be continuing their education, with intense support from the school and other health professionals. Setting arbitrary targets will only discourage schools from taking on additional pupils with serious medical issues, as those schools that do will be unjustly labelled and blamed," Mr Lightman said.
Attendance statistics are published twice a year and Ofsted uses them when inspecting schools.
In a document detailing the changes, DfE officials say they recognise the reforms will mean children could be classed as being persistently absent if they have had "relatively minor illnesses".
"Of course there are pupils who are off school for long periods of time for medical reasons and it is important that the Government is not seen to be heavy-handed with families going through difficult times. Nor should schools be penalised for the absence of genuinely sick children," the document says.
Alison Ryan, education policy adviser for education union the ATL, said the changes could encourage teachers to exclude children with poor absence records, or take them off the school roll.
"This doesn't encourage teachers to look for the causes of persistent absence, and it could cause divisions between parents and schools," she said.
"It doesn't do anything positive and doesn't help teachers fight the problem. What they really need is funding for support services and extended schools activities, which really help improve children's attendance."
Staff at Bishop's Hatfield Girls' School in Hertfordshire are supporting a Year 10 pupil with severe diabetes, who has only managed a 10 per cent attendance rate this year. Despite this she has taken five and a half GCSEs this summer, some while in hospital, and is predicted to get A grades.
"This new target shows no recognition of what schools are actually doing to support those absent for medical reasons. I know there are a lot doing similar work to us," said headteacher Theodora Nickson.
It's still OK to use the force
The final shortened behaviour guidance for teachers was also published last week by the DfE.
It says schools should not have a "no touch" policy and reminds teachers they have the legal power to use reasonable force to remove pupils from the classroom or to stop them leaving. It says heads can search for items such as alcohol, drugs and stolen property.
NUT general secretary Christine Blower said this could lead to an "unhelpful change in the schools' culture if airport-style security measures and frisking are seen as normal".