Only a very few international schools have qualified teachers. The rest hire anyone who's willing to chore for the minimum salary, and do a job with no future.
Students are sent there for two major reasons:
1.They couldn't enter a proper gov school.
2. They just need English to continue their businesses, especially Muslims.
But about English, except for the notes in broken English, they use Sinhala at school. Only a very few teachers actually speak English at all times.
All in all, if you don't go to a proper international school like CIS you end up with broken English (if you don't speak it at home, especially village businessman's kids etc), and a hatred towards gov schools.