One Angry Mother!

My son had an incident last week in which he was bullied. It wasn't that big a deal, but it revealed that at his religious school, his class had been left alone for a while. At his age (10), the opportunities for mischief when a group of kids are left alone are many, so along with e-mailing the mother, I sent an e-mail to the principal of the religious school questioning why the kids were left alone. It came out that school's policy is that the kids are never left alone, but that on this day, one teacher needed to leave for a family commitment and the other was unaware of the timing of this.
Anyway, I saw the mother yesterday, and she was pissed! She claimed I should have called her and talked about this with her. Perhaps she's right, but I still would have wanted to get the unattended chidren issue addressed. So, this could not have been properly addressed without involving the school. Anyway, her response was something along the lines of "I'll never help you again."
Still, the mother claimed that seven other children were involved, based on what I heard yesterday, it was two others, but often the number of children involved is subject to some interpretation...Are laughers involved? What about commenters? And so on.
Was I wrong? I don't really think so, though I'll acknowledge that I might have better handled the interpersonal side of things.