At the Boatkhali Kadambini Pre-primary School on Sagar Island in the Sunderbans, classes stop for five-six days each, twice a month, during June to August. Sea water invades the classrooms to a height of one-and-a-half feet, rendering teaching impossible.

"It recedes after two-three hours, but it happens twice a day for a few days, so no classes are possible. Children only come for the midday meal," a teacher at the school said.