Second-level schools are increasingly concerned about competition for students, with nearly half experiencing difficulties attracting the best pupils.
A new survey of school principals shows that high-achieving students are not necessarily enrolling at their local school, but are instead pre-selecting the schools they wish to attend.
One effect of this is that fee-paying schools, as well as larger schools and those in urban areas, are frequently oversubscribed. By contrast, vocational schools and designated disadvantaged schools are very unlikely to be oversubscribed.
Some 44 per cent of the 567 post-primary principals who responded to a postal survey described themselves as suffering to some degree from "negative selection" or "creaming off" of students.
This means they struggle to attract the most academically-gifted students, according to the survey commissioned by the National Council for Curriculum and Assessment (NCCA).
Of these, vocational schools reported more of this "negative selection" than other schools.
The revelation that nearly one in every two schools is struggling to attract students of high academic ability may explain why more than 20,000 places in the free second-level school sector in Dublin were unfilled last year.
It indicates that parents are deciding to send their children to schools outside their locality.
This is further underlined by the fact that the demand for places at almost all of the 37 private fee-paying schools in the greater Dublin area has increased sharply.
According to one of the co-authors of the report, Dr Emer Smyth, of the Economic and Social Research Institute (ESRI), the findings support research which showed that half of second-level students do not attend their local schools.
"There is quite a lot of selection of schools going on by Irish parents. Some parents are choosing not to send their children to vocational schools as their first choice."
The majority of over-subscribed schools consider it very important to have an older sibling of a new student at the school when determining criteria for enrolment.
Dr Smyth said this means that parents need to be aware of the implications for their younger children of sending their children to a particular school.
The report also reveals almost half of the participating schools had experienced some decline in their student intake over the previous five years, with just one-fifth increasing their student numbers.
Vocational schools were most likely to experience a decrease in student intake, followed closely by secondary schools for girls and secondary schools for boys.