The Lost Twin

Middle grade mysteries are just so enjoyable to me. This one is set in a boarding school, which is a trope I've loved all my life, and it features a main character that gets to grow a lot, a sidekick who is frankly adorable, and it's set in the 1930s, so there's the historical aspect as well.

Also, I 100% pictured Mrs Fox as Matron from “Wild Child”.