A primary school has been oversubscribed at an early stage of the Primary 1 registration exercise - and may even need to go to ballot.

CHIJ St Nicholas Girls' School in Ang Mo Kio had 97 children vying for 92 spots when registration for Phase 2A1 closed yesterday.

This is the second of the seven registration stages. It is the first time a school has been oversubscribed so early since Phase 2A1 was introduced in 1999.

It is for children who have a parent belonging to the school's alumni association for at least a year or on the school advisory committee.

In the past, all applicants at this stage have found a place.

The school had already set aside 40 of its 210 Primary 1 vacancies for the later 2B and 2C phases, under a rule introduced last year.

The squeeze may be due to more parents joining alumni associations to give their children a better shot.

Former pupils who are not members of the alumni association can register their children only in the later Phase 2A2 - limiting their children's chances of getting in.

Last year, CHIJ St Nicholas was one of four oversubscribed primary schools to hold a ballot at the third Phase 2A2. It was the first time in nearly two decades that balloting was held at the Phase 2A2. In 1999, the old Phase 2A was split into 2A1 and 2A2. The last time a ballot was held so early was at Anglo-Chinese School (Barker Road) in 1995.

After Phase 2A1 closed yesterday, a total of 19 schools were left with fewer than half their total vacancies - down from 25 last year.

From Phase 2A1 onwards, when the number of applications exceeds the vacancies available, places will be balloted based on the home to school distance criterion.

A parent who wanted to be known only as Alan yesterday registered his son at Henry Park Primary, as he has been an alumni member since 2011. "I know that my kids would be in good hands," he said.

Results for the Phase 2A1 will be out tomorrow.