The union says that local government cuts mean a typical educational welfare officer handles a caseload of 300 children. Some 2,500 such officers are needed to solve the crisis, which is leading to an increase in juvenile crime.
"On an average day, 80,000 children and young people are out of school, often roaming the streets, exposed to juvenile crime, drug abuse and illegal child employment," said John Findlay, Unison's national officer for social services.
"Without a huge increase in resources, the problem will go unchecked. We need trained and experienced staff to work with the children concerned, their families and their schools to identify and deal with the underlying reasons for truancy.
"There is a crying need for further detailed research if we are not to store up social problems for the future."
He claimed Government figures grossly underestimate the true level of truancy. This is because schools are under pressure to play down the number of pupils who play truant, and the present method for recording absences is open to abuse by staff and pupils.
The recorded figures show an average of 16 half-days lost each year by each truant, with more than 130,000 children truanting. But children may be present for registration, only to play truant later.