Luna's the only one at the school that knows, because she's the only one the Teacroft Toflies know will listen to the secrets they glean from the stars.
The House Elves no doubt have more than an inkling between themselves, with all the morning teas for two that they've put together, but they don't pry. Luna surprises the two of them with an anniversary present, fluffy furred cats chasing multi-hued stars across the wrapping paper, and skips off to class while they're recovering from their shock.