My daughter took the indirect route. She told other kids about the incident, who confronted the culprit.
“Did you throw a ball through her window?” one said, loudly.
“Yeah.”
But no mention of confessing.
Then, the “girlfriend” of the culprit was approached. “He is such an idiot,” one child said to her.
I am not sure what it means to have a girlfriend in 6th grade, but I’m told that in this case, the kids talk on the phone at night. We are hoping the issue came up in last night’s call, prompting some sort of epiphany about moral character and thus, a break up, leading to sad faces and inquiries from said mother and, ultimate disclosure. Of course, I could just put the note in her mailbox. Am I avoiding this? I think so. I will give it until 5 pm.
Pamela Gwyn Kripke 
If the boy’s mom is going through tough times, why don’t you take the moral decision to not add to her burden. It was a mistake. The boy apologized. He obviously still feels badly. Perhaps you could afford to fix the window better than she can at this time and most likely, once you tell her about the incident she will feel awful and will expect to pay you for it. So how about the moral epiphany be that you accept the boy’s apology and let it go.
Consider that by leaving the mother of the boy out of the loop about the incident you are not saving her an additional burden but depriving her of seeing how her son handles a moral/social situation and the opportunity to parent him accordingly.
The boy apologized, however his actions indicate it is not resolved. The boy is a child and he needs to have appropriate behavior modeled. Offering restitution is not a punishment but a way to show responsibility. It doesn’t have to be about money. “Sorry” could be adequate or maybe a little “working it off”, it just needs to be an open, collaborative decision that involves his parent.