I intended the story to be set in a time (60s or 70s) when such classroom discipline was routine, and informing parents was not expected (but might be done if the school wanted the parents to back them up with an additional punishment). So it wasn't a matter of the teacher "getting away with" anything, or having her hide it from her parents to protect him.
That said, I did hesitate about having the teacher essentially encourage her to lie (by omission) to her parents. But in the end, I wanted Sarah to "get away with" just the one spanking, which pretty much required her parents to not know about it.
Also for those who expect to see young Sarah over the desk again soon, alas, I fear you will be disappointed. In my mind Sarah learned her lesson that day (what? the spanking actually worked?) and will manage to avoid her parents finding out, and stay out of trouble going forward. I think of Sarah's story as a "good kid crosses a line without really intending to" kind of story, rather than a perpetual trouble-maker story/series.
Of course, I left a lot unsaid in the story, so feel free to imagine whatever alternate post-scripts turn you on, including perhaps the post-script where her parents *do* find out, and spank her again that evening.

And yes CrimsonKidCK, I definitely identify more with the kids than the adults in spanked child stories. Which is probably why most of the kids I write about are girls, though I do mix it up from time to time.