Many pupils are not covered by school medication policies
The health of many children with type 1 diabetes is threatened due to poor care in primary schools, say campaigners.
Diabetes UK claims 6,500 pupils in England are not covered by school medication policies which ensure they get insulin injections when needed.
This means parents often need to visit during the school day to administer injections.
The charity wants ministers to force school inspectors to keep checks on the care of children with diabetes.
Thousands of children in the UK have type I diabetes, caused by the sudden loss of insulin-producing cells in the pancreas.
Most are wholly dependent on regular blood testing and insulin injections to avoid severe complications - which can include diabetic coma and even death.
In the long term, poor control of diabetes can increase the risk of heart disease, stroke, kidney failure and amputation.
There is already evidence that blood glucose levels are poorly managed, with research in 2007 suggesting that only one in five children had their levels properly under control.
Injection regimes are a particular problem at younger ages, when children do not have the skills or confidence to carry out testing and injections.
Diabetes UK says its research suggests only 16% of primary schools in England have medication policies which allow them to administer insulin.
This can mean that treatment regimes are changed to fit around school hours, or that children are not cared for properly in school.
The charity said that parents had told it of instances in which children had to inject insulin in school toilets, or, in one case, wait outside the gates for the arrival of a nurse.
Douglas Smallwood, its chief executive, said: "It is unacceptable for medication regimes to be changed to fit around school hours.
"Children must have the most appropriate treatment for their diabetes and be properly supported in managing their condition.
"Where parents have to step in to give insulin injections during school hours, we hear all too often that they are unable to work because they have to go into school every day - this can put heartbreaking strain on families both financially and emotionally."
He said that the government needed to strengthen legislation to include the well-being of children with long-term conditions - and to make sure that school inspections included the presence or absence of a medication policy.
A spokesman for the Department for Children, Schools and Families, said there was no legal duty on schools to administer medicines, but most assisted on a voluntary basis.
In Scotland, a similar survey suggested just one in ten primary schools had a policy on giving medication and were prepared to follow it.