This happens from time to time with my kids' homework. For example, today their 5th grade math homework was to solve some algebraic equations. One of them required an understanding of negative numbers and the fact that negative times negative equals positive. This has never been taught in their school yet.
Their school grades homework, so an answer needed to be provided. I told my kids the simplest way to make the numbers positive. I told them that if their teacher asked how they knew to do that, they could tell him I helped them with that. I wondered if I should write a notation "mom helped" but decided not to.
So I'm curious - for kids in upper elementary / middle school, what is your policy with assignments that require not-yet-taught skills?