Dear Natalia,
I really recommend the book "Of Woman Born" by Adrienne Rich. Not all of it may speak to you, but the author writes beautifully and compellingly about the absurd expectations that our society has for mothers, and the limitless blame that all mothers experience. It's as though every kid must be above average in every way, and if a kid is just a kid -- with an unwiped nose or a public tantrum (mine always seems to save his for the grocery store), of if a child isn't perfectly socialized by the age of 5, than that kid must have a "Bad Mother." And we feel equal measures of guilt and anger. It's completely normal.
All that said, I want to, as gently as I can, recommend trying to see things from the other mom's perspective as well. Bullying is a huge deal. I don't think it should even be called bullying; that trivializes it, and the effect it has on children. I was bullied pretty much nonstop from kindergarten through 5th grade, was completely friendless, etc. The teachers did absolutely nothing, never reported it. More than anything else in my life, that has shaped who I am -- smart (sometimes anyway), and with a critical, questioning attitude, but also painfully shy and reflexively mistrustful. My biggest fear as a mom is that my son will go through that, and I want more than anything in the world to prevent it.
So, as the other mom approaches you, I imagine she's probably shaking and not sure what to say; she's probably feeling like a "Bad Mom" for the fact that this happened to her child. The urge to defend -- it happens simultaneously, on both sides.
I hope this made some sort of sense, and I wish you the best,
Mira