If you are looking for a very good International School, outside of crowded Colombo City, I would suggest the N.I.C.E. School in Negombo (Kadirana), which is a beach resort area close to the Airport.
This is a very good school, with small classes, run by the parents themselves (mostly expats), with very high standards.
Negombo is a wonderful place to live, very green and un-crowded, but with plenty of shopping and things to do and see.
Good Luck!