Children who have asthma, epilepsy or diabetes are being put at risk in the classroom because of falling numbers of school nurses, health experts have warned.
According to new data published by the NHS, over 550 school nurses were lost over the last seven years.
The fall has gathered pace in recent months, with more than a hundred posts lost so far this year.
Almost a quarter of 11-15 year olds in England report have a long term illness or disability, including asthma, diabetes, epilepsy and arthritis.
The Royal College of Nursing (RCN) says the loss of school nurses is leaving teachers without vital training and pupils without necessary support.
It is calling on the government to ensure local authorities have the funds needed for fully-staffed school nursing services.
The continued deterioration of services could leave pupils with health conditions unable to attend mainstream school, the RCN has warned.
The number of school nurses has fallen despite statutory guidance from the Department for Education in 2014, which stated that all children with health conditions should be supported to go to school.
"Children with conditions such as asthma, epilepsy or allergies could experience a life-threatening emergency at any time," said Fiona Smith, RCN professional lead for children and young people's nursing.
"Without the right training, guidance and support from school nursing services, teachers could be completely unprepared for this kind of situation - putting children's lives at serious risk," she added.
"It is time the Government wakes up and realises the hugely detrimental impact these cuts are having to our children and our society. School nursing is a critical service and it needs to be treated as such."
Kevin Courtney, General Secretary of the National Union of Teachers, said:"The drop in the numbers of school nurses is yet another symptom of the school funding crisis.
"It is simply untenable that head teachers should have to continue to struggle on with a situation that is so negatively affecting both the education and well-being of our children and young people."
A Department of Health spokesman said local authorities make decisions about public health funding but more investment is being made: "School nurses play an important role in supporting the health and wellbeing of young people.
"Local authorities are best placed to make choices about services for their community which is why decisions about public health funding sit with them.
"To help, we are investing more than £16 billion in local government public health services over the current spending period, and will continue to support schools in their duty to make arrangements for pupils with medical needs."