I would assume it was a mithered teacher/ta who remembered at the very last minute
That's the thing though, isn't it? If you're a teacher, it's your responsibility to be organized enough that these sorts of things don't creep up and make you "mithered". At least that's the impression I'm getting from the teacher training course I'm currently doing.
All that aside, I'd be seriously side-eyeing any teacher who made it as far as getting put in front of a class and didn't understand simple homophones. How can you teach them if you don't know them?