The number of school nurses in the UK needs to double, the Royal College of Nursing has said.
School nurses have a wide-ranging role
On average school nurses cover at least 10 schools and there are "unacceptable" shortages in some areas, said the RCN.
Last year, the government pledged each secondary school and cluster of primary schools would have a dedicated nurse.
But the union said that primary care trusts needed ring-fenced money to ensure that happened.
The RCN, which begins its annual conference in Harrogate on Sunday, says there are currently fewer than 3,000 school nurses covering the UK, each caring for an average of 2,400 pupils.
It welcomed the pledge to bring in more school nurses.
However, a spokeswoman said whichever party formed the next government would have to ensure primary care trusts (PCTs) were given dedicated funds to implement the policy, because of the myriad of other demands on resources.
She added the nurses could be recruited only if more people joined the profession.
The RCN surveyed more than 1,200 school nurses from across the UK.
It found that only 10% of those working in the state sector - three-quarters of those surveyed - felt that there were enough school nurses in their area.
Nearly two-thirds of all those surveyed thought that their workload was too heavy.
And the majority thought they were poorly paid for the demands and responsibilities of the job.
Dr Beverly Malone, general secretary of the RCN, said: "It's clear from our survey just how few school nurses we have.
"We know that in many parts of the UK children do not have access to an adequate school nurse service, and this is simply unacceptable."
She added: "School nurses of today are not just about sick bays and nits.
"Their job covers a gamut of responsibilities from immunisation, health promotion and child protection through to counselling, sexual health and drugs education.
"They also play an important role in social inclusion such as working with children with special needs and promoting educational attainment."
Ms Malone added the RCN was urging the next government to strengthen the role of the school nurse, develop a clear strategy to double their numbers and make funding available for every school nurse to have the opportunity to access specialist training.
Extra nurses pledged
A Labour spokeswoman said: "PCTs have been given cash to ensure there's a school nurse for each secondary school and for each cluster of primary schools.
"It is now up to them to implement that policy."
Shadow Health Secretary Andrew Lansley said: "We have pledged an increase of 30,000 extra nurses by 2010 and our public health policy, with a dedicated health budget, will enable school nurses to be confident of financial backing in the future."
Paul Burstow, Liberal Democrat health spokesman, said: "The school nurse has a key part to play in winning the hearts, minds and tastebuds of the next generation. The problems with obesity in our schools will only be solved by a keen focus on prevention.
"The Liberal Democrats would recruit and retain thousands more nurses."