I live in Werdenberg, some 30min./35min. south of Sankt Gallen, and my daughter attends the local international school (it's called ISR, International Schoor Rheintal). It's fairly expensive (~18K/yr) but an excellent school, and my 6-year old daughter loves going to school every day. The classes are small groups (1st grade has only 12 kids), they have a main teacher and an assistant teacher, young, modern people with an open mind, a great programme. Check them out at their website:
www.isr.ch
If you need to live close to St. Gallen and you're tempted to send your kid to this school you can always choose a decent city halfway between Sk. Gallen and Buchs, like Altstätten or (smaller but closer) Oberriet, SG.
Just my 2 rp.