Don't Singaporeans going to international schools have to get it approved by the government first? I'm pretty sure you just apply, get placed on the waitlist, and they will notify you when you have a spot.
I heard that the Singapore is changing their policies regarding Singaporean citizens in international schools. The new East campus has greatly increased the total possible students in highschool which should decrease any difficulties of obtaining a position. If I were you, I would directly contact the school about the statistics.