My kids are 12 and 13. If any of their teachers hear from us, it's pretty bad. In fact, our son was punished inappropriately once which was the only reason I've ever gone to the school to complain. That time, the headmaster met us with an apology straight out of the gate. One of the reasons he did this was, according to him, the fact we didn't step in very often, and that spurred him to investigate more deeply. For that matter, our kids are responsible for discussing any planned absenses, grading error problems (thankfully these seem to have dried up recently), or other problems of this sort, and we hold them accountable in the end. We wouldn't let them get completely wronged, but I've only regretted not stepping in on one occasion. It's been that way at our house for two and three years now. It pisses me off a bit to even read about someone contacting an instructor at a college over their legally adult children. How can any such behavior be considered healthy.
My dad, to best of my knowledge, even when he was my principal, stepped in twice. And one of those times someone else inatiated it.