Children who have asthma, epilepsy or diabetes are at risk in the classroom because of dwindling numbers of school nurses, experts have warned.
The Royal College of Nursing (RCN) is calling on the Government to ensure local authorities are provided with the funds needed for fully-staffed school nursing services.
It says the loss of school nurses is leaving teachers without vital training, and pupils without necessary support.
The continued deterioration of services could leave pupils with health conditions unable to attend mainstream school, the RCN has warned.
According to data published by the NHS this week, more than 550 school nurses were lost between May 2010 and May this year, almost a fifth (19%) of the total NHS workforce in England.
The fall has gathered pace in recent months, with more than a hundred posts lost so far this year.
Lisa Thurston's son Owen developed epilepsy when he was at primary school and his condition became more severe as he got older.
She said: "The school's support in Owen's care was absolutely critical. When Owen was diagnosed in 2010, the school nursing team provided free training to ensure the school staff had the skills in emergency care to help Owen if and when he needed it.
"We were very lucky that Owen got this crucial care, but diminishing school nursing teams will have a serious impact on similar children with health conditions."
She added: "As a parent and a school governor, it is frightening to see the difference between statutory guidance and what care is actually available.
"Every child deserves to be safe at school and without the right health care staff I just don't think this will be the case."
Almost a quarter of 11-15 year olds in England report have a long term illness or disability, including asthma, diabetes, epilepsy and arthritis.
Despite new statutory guidance from the Department for Education in 2014, which stated that all children with health conditions should be supported to go to school, the number of school nurses has fallen from 2,987 to just 2,433 full time NHS posts in England.
Fiona Smith, RCN professional lead for children and young people's nursing, said: "Children with conditions such as asthma, epilepsy or allergies could experience a life-threatening emergency at any time.
"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 wellbeing of our children and young people."
A Department of Health spokesman said: "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."