Children are deterred from using school toilets in secondary schools because they are dirty - and occupied by smokers and bullies, a survey warns.
A quarter of the 300 children surveyed by experts from the London School of Hygiene and Tropical Medicine said they avoided toilets if at all possible.
Speaking before Friday's Global Handwashing Day, the scientists said facilities were "dirty and inadequate".
One teachers' union said toilets needed to be clean.
More than a third (36%) said their toilets were never clean, with 42% saying soap was only available sometimes, and almost a fifth (19%) said there was never any soap.
Nearly 40% of secondary school girls reported ''holding it in'' so they didn't have to go to the toilet.
And 16% of secondary school boys reported "bad things" happening in the toilets, making them wary of going in there.
Around 150 primary school children were also questioned in the survey, but they reported far fewer problems with their toilets.
Dr Val Curtis, director of the London School of Hygiene and Tropical Medicine's Hygiene Centre, who led the research, said action needs to be taken to protect the children.
"Our survey has found hand washing in Britain's schools to be less than ideal," she said.
"It would be easy to blame laziness on the part of the kids for this state of affairs, but clearly the problem lies with inadequate and dirty facilities, particularly in secondary schools."
"There's a strong economic case for investing in good hand washing facilities in our schools. Britain's 12m cases of norovirus, gastroenteritis, MRSA, E.coli and now swine flu infections are mainly down to dirty hands.
"Our children deserve better than to be exposed to avoidable illnesses because we are not doing enough to provide safe, clean toilets in our schools."
The children quizzed said they wanted to see toilets cleaned more frequently, soap and toilet rolls always available and CCTV cameras removed to protect privacy.
Christine Blower, general secretary of the National Union of Teachers, said school toilets needed to be clean.
"The provision of clean well maintained toilets, and by this we include adequate soap and toilet paper as well as locking doors, is an indication to children and young people that they are respected by their schools," she said.
"In particular adequate provision for menstruating girls is essential. In these circumstances pupils are obviously more likely to respect the facilities."
A spokesperson for the Department of Education said: "We urge schools to take a common sense approach to keeping safe".