It's set in 1910.
> For while the sisters prized proper learning, they also prized good manners, thoughtfulness, and care for others, and the girls learned both algebra and needlework. Moreover, they took in children whose parents were abroad and needed somewhere to spend the holidays. Now, some thirty years later, in the autumn of 1910, the school had a waiting list, and those girls who went there knew how lucky they were.
(p. 1, ch. 1)