More children are starting school in Northern Ireland before they are toilet trained, a study showed.
There has been an increase in the number wetting or soiling themselves, the survey for the Association of Teachers and Lecturers said.
Some 62% of primary school staff in the UK-wide review have noticed a greater problem.
Anne Millis, Northern Ireland president of ATL, said: "Having to deal with increased numbers of pupils who have not yet been toilet trained puts extra pressure on education staff when they already have enough pressure on them.
"Schools need to give staff clear guidance on how to deal with toileting accidents so that they know what they are allowed to do and who should be dealing with an incident.
"It is also important that education staff feel that have support from their school nurse or head, and that they know where to obtain guidance should they need it."
The proportion of teachers noticing an increase in incontinence increased to 71% UK-wide amongst those working with three to five year olds - those in primary 1 in Northern Ireland.
ATL surveyed 848 education staff working in state, independent and academy schools in the UK during October and November 2011.
Those asked felt that the main reason for the increase in the number of children wetting themselves at school was due to parents not toilet training their children before they start school.
The union said legislation had led some schools to believe they can no longer refuse to take these children.
Just under four in 10 (38%) respondents stated that their school has no written policy for dealing with childhood continence problems. Around 35% said their school has no written policy for dealing with childhood toileting accidents.
Some 38% of schools said they do not provide written information to parents of school starters about ensuring their child is toilet trained before starting school, while 36% said they do.
A Derry-based teacher and member of the senior management group said: "This is a major problem for us - over 45% of our nursery children are not toilet trained when coming into nursery when they are three years old. We also have children who soil and wet a great deal even in reception.
"Our parents just have no idea when and how to toilet train their children. We are having to put on a workshop to support them."
A teacher from a controlled school in County Antrim said nappies have been designed to absorb large quantities of liquid.
"Children do not feel wet or notice any discomfort and this seems to delay their urge to be free of nappies. Pull-ups are similar," she added.
ParentsOutloud, which is campaigning for greater flexibility in Northern Ireland's school starting age, said: "Northern Ireland is the only territory in Europe where children are required to commence formal schooling as young as four years and two months," said Liz Fawcett, Northern Ireland representative for ParentsOutloud.
"While we are very supportive of efforts to encourage parents to toilet train their children at the appropriate age for that child's needs, we are not surprised to learn that some children are not fully confident in using a toilet when they start school.
"Indeed, expert information suggests it is not unusual for children of four or five to require toileting assistance, particularly with regard to bowel movements and in relatively unfamiliar toilets."