Mr. Lacey and Miss Winter are having an affair.
He is portrayed as a sensible person who wants his daughter to become one too, so why else would he let this complete airhead teach her for years?
- Alternatively, Miss Winter is having an affair with Mrs. Lacey. Or with both of them.
Darrell has a crush on Gwendoline.
There's a scene in the first book where she wants to braid Gwendoline's hair to help her to look more proper something none of the other girls would have done. And whenever she's trying to be nice to Gwen and her attempts fail she gets extremely frustrated, probably because it shows her that her crush is hopeless and one-sided. The scene where she loses her temper and slaps Gwen repeatedly for holding Mary-Lou underwater could partly be driven by that reason, since it was such a cruel thing to do and another sign that they would never work out (because there's no way Darrell could justify such a thing).