A 'demographic bulge' leaving parents short of school places for their children isn't going to last, a principal has said.

It comes as nearly three out of four parents are unable to secure places for their children at Ireland's most oversubscribed second-level schools.

About half of all secondary schools across the State are oversubscribed, according to The Irish Times, with the highest concentration in the Dublin area, commuter belt counties and some large towns.

Alexandra College Principal Barbara Ennis told Newstalk Breakfast she believes the surge for school places will reduce.

"There's been a demographic bulge over the last three years," she said.

"That's due to lessen over the next coming years as the birth rate for that age group is not as high as for the current age group.

"That's the main reason because of the demographic bulge".

Students doing exams in a classroom. Image: Prajuab Chaipimpa/Alamy

Ms Ennis said their admissions policy doesn't give preference to people in the locality, which she said is a problem.

"It is, it most definitely is [an issue] - but at the same time we have to follow the needs of the school," she said.

"It being a boarding school it's essential that we fill our boarding places first.

"Our admissions policy is boarders first, then our junior school students, then members of the Protestant faith and then other applicants after that.

"After that we have applicants who apply in the local area and people who have come to Ireland from abroad as well will apply and they will fall in under heading number four."

'A school that's too big'

Ms Ennis said she believes making schools larger is not the answer.

"I think it's a temporary problem and the question of expanding your school can always be asked - but that's not something that we want to do," she said.

"We already have a big school; we've 650 in the secondary school and we do not want to expand... because it changes the whole atmosphere and feel of the school.

"If you increase, forgetting about the effect it'll have on the atmosphere, you're then going to be left with a school that's too big when the demographic bulge begins to recede," she added.

The most heavily in-demand schools are in south Dublin where as little as 20% of applicants can be successful.

Main image: Students in Dublin, 4-9-20. Image: Sasko Lazarov/Rollingnews.ie