The three home-grown international schools cannot seem to expand fast enough to keep up with demand for places from Singaporean parents.
Anglo-Chinese School (ACS) (International), St Joseph's Institution (SJI) International and Hwa Chong International School have expanded their facilities in recent years to take in more students.
Yet, the demand for places, especially from Singaporeans, keeps growing, despite annual fees of more than $20,000.
ACS (International), which expanded its facilities at its Holland Village campus last year, has already seen its enrolment rise from 800 two years ago to 1,000 now.
When more classrooms are added by the end of this year, it will be able to take in up to 1,200 students. Half of the places there are taken up by Singaporeans.
At SJI International's high school in Thomson Road, enrolment has risen from about 800 two years ago to 977. It is adding classrooms this year and will be able to take in 1,027 students.
Hwa Chong International School in Bukit Timah Road has seen numbers rise from 600 two years ago to 750. When the third phase of its building programme is completed in the middle of this year, enrolment is set to hit 800.
The three schools are privately funded but are under the Ministry of Education. This means they follow the bilingual education policy, but can design their own curricula and set class sizes and fees.
Fourth-year students take the International General Certificate of Secondary Education exam, similar to the O levels, and then study for the two-year International Baccalaureate (IB) diploma.
When the Government gave the go-ahead in 2004 for ACS, SJI and Hwa Chong to open international arms, there were doubts on whether Singaporeans would want to enrol their children.
Officials from the schools said the rapid growth is the result of the established brand names and different curricula, particularly the IB programme, which is becoming popular among parents.
At SJI International, 62 per cent of places are filled by Singaporeans. All places for this year's Grade 7 classes, equivalent to Secondary 1, were taken by last July. Places for Grade 11, equivalent to the first year of junior college, were filled earlier than before.
Mr Bradley Roberts, head of the high school, said: "The school is completely full at all grades."
ACS (International) chairman Tan Wah Thong said the school is at its maximum size, but parents from Singapore and elsewhere are still asking to enrol their children. He said the school will have to look at other locations to expand.
"The ACS name is a draw, but because we have no streaming and allow all students to move up to JC to take the IB exams, it is attractive to parents," said Mr Tan.
Hwa Chong International principal Bob Koh said many local students blossom in international schools. "They have more space to grow and more individual attention because of smaller classes."
Parents said they liked the IB programme's smaller class sizes of 20 to 25 students, and the fact that there is no streaming.
But two of the eight parents interviewed also said they picked an international school as their children failed to get into the Integrated Programme schools or JCs.
Shop manager Esther Tay, who hopes to send her son to SJI International next year, said he suffers from exam anxiety despite doing well academically. "A school that does not stream students will suit him better. I was shocked when I saw the fees, but in the end I decided they would be worth it if my son is going to be happier."