Why are Santha and Premila sent to the english school?

They're sent to a British school mainly for practical reasons. Santha and Premila used to be home-schooled by their mother. She did this for two reasons: first, because she didn't think that Indian schools were good enough; and second, because she felt that her daughters would lose their identity in a British school. But when their mother falls ill and their father is away on a long business trip, Santha and Premila are sent to a British school after all.
After the girls start school, their mother's worst fears are realized. At school, the English headmistress—who can't speak the native language—changes the girls' names to the more English-sounding Cynthia and Pamela. The girls are forced to wear the same kind of uniform as the British children; they're also forbidden from eating Indian food. To add further insult, the girls are told that Indian students must sit with one desk between them when taking a test. The reason? Because, according to the headmistress, "Indians cheat."