Teachers are dealing with pupils who soil and wet themselves because parents are sending them to school without being toilet-trained.
Bob Martyn, Oxfordshire branch secretary of the Association of Teachers and Lecturers (ATL), said he had been approached by teachers at half a dozen schools who have had to clear up after children.
Speaking at the union's annual conference in Torquay next week, he will say that pupils should either be sent home until they are able to use the toilet, or schools should be funded for more support staff.
He said that teachers accepted that some pupils with special needs may have difficulty using the toilet, and in most such cases, the local authority provides specialist support.
The difficulty comes, he said, when parents cannot be bothered to teach their child to use the toilet, instead sending them to school in pull-up nappies in the expectation that they will be taught at school.
Ann Nash, a Bradford health and safety representative, said parents should be told to keep their child at home until they are toilet-trained.
Teachers are forced to work in pairs to clean up children, she said, for fear of being accused of abuse.
At one school she worked at, a third of entrants were not toilet-trained. "Parents think it's not their responsibility - they think it's the school's problem," she said. "A teacher becomes a nappy-change service, not an educator. Parents say it's easier to put them in nappies. Well, we've all been parents; we know it's hard. But that's your job."
Mary Bousted, ATL general secretary, said: "It may be an issue that people laugh at, but three- to five-year-olds should not be turning up to nursery or school in nappies."
Her concerns echoed those of John Dunford, the general secretary of the Association of School and College Leaders, who said last weekend that schools were being expected to cope with young pupils who can barely hold a conversation or are able to use a knife and fork.
Linda Taylor, an experienced foundation teacher in Oxford, has been off work with stress after having to deal with a special needs pupil who filled his nappies five times a day. Her school had hired support staff to change the boy's nappies, but she said his difficulties were still profoundly disruptive.
She does not blame parents, but puts it down to the fact that more children start school younger now.
Lesley Staggs, an early childhood consultant, said most parents do care about toilet-training. "Some children do take longer than others," she said. "When I was a nursery teacher, I changed children's wet pants and it wasn't a big deal."
Other debates scheduled for the ATL conference include whether the school leaving age should be lowered to 14. Members will also call for a Royal Commission to investigate why so many children are unhappy, and for the scrapping of homework at primary school. They will propose tough laws against bullying, and ask for a "fit and proper person" test for school proprietors.
Ban primary homework, page 12
Leading article, page 32.