Dumfries and Galloway says it is "virtually impossible" to know if more children are working illegally compared with three years ago.
A report to the council's education committee, prompted by a press report citing a Paisley University study, pointed out that the research dates from 1992-94 when pupils attending three of the council's schools were interviewed. Without new research it would be impossible to estimate the current position, the report states.
Returns from Dumfries and Galloway's 16 secondary schools covering the last two sessions show a slight increase in the number of work permits issued by headteachers, up from 59 to 60 for boys and from 70 to 88 for girls. Most were in second, third and fourth year.
But last week's report by Christine Dignan, the council's head of secondary education, noted: "What is much more difficult to ascertain, without very detailed research, is how many youngsters might be or might have been employed without having obtained a work permit, what type of job these youngsters might be doing, their hours, conditions of work."
School staff are asked in their returns to the council whether they are aware of any pupils working without a permit. "Not many, but impossible to quantify" is typical of the responses.
Social education classes are used to warn pupils they need a permit before taking on part-time work. Schools also remind parents in writing and of the need to balance schoolwork against jobs.
The council has placed newspaper advertisements to remind businesses of their obligations when employing young people and environmental health officers are told to watch for breaches.
The Scottish Office proposed two years ago that national regulations should be introduced but local by-laws remain in force. In Dumfries and Galloway, employment is limited to two hours on a school day but not before 7am or after 7pm. It also sets out prohibited occupations such as helping in amusement arcades and slaughterhouses.
The Paisley University study three years ago involved 259 children and found that more than a quarter started work before 7am and two out of three finished after 7pm. This was "worryingly high", the council stated. Only a third had a work permit and a fifth said they had had an accident at work.