Today they will present evidence of the growing problem to Welsh Assembly members. Schools have been forced to deal with a 10-fold increase in pupils from the north of England and the West Midlands in the past three years, according to Mike Mulvaney, head of Pendorlan junior school, in Colwyn Bay.
Mr Mulvaney has admitted 43 new children since September and was last week asked to take six more in the space of four days.
He said: "The image is of lovely green pastures and golden sands but we have high unemployment, low income and areas of deprivation before you even look at the problem of a transient population.
"One or two children who are disaffected or who may have been excluded from English schools can create real problems."
The head also claimed that the effect on small, Welsh-speaking village schools could be devastating.
At Foryd primary school, Colwyn Bay, 11 out of 15 special needs pupils have come from urban England.
Head Diane Wootton said low-cost housing and the scenic landscape attracted people in "considerable numbers". But, she said: "We do not have the resources to meet their needs. Special needs funding should follow the child."
A Government-funded report published last year found education standards were being hampered by high pupil turnover.
The study found one in seven local education authorities had schools where a third of pupils left and were replaced by outsiders in a single year. A fifth of authorities believed constant movement of pupils damaged achievement.
At present, central Government grants to councils take account of how many pupils are eligible for free school meals but not of pupil turnover.