I do hope you all realize that different places do operate differently, some are more employee friendly than others. It's just a fact of life. ...and yes, theoretically, if you didn't do anything wrong, then, no, you shouldn't have anything to worry about, but there is still the stress of being involved in an investigation.
...and yes, my last employer, would, and did, have people who would contact you at home. It would not be an inquisition, but they would, definitely, try to worm whatever info they could out of you. They would be very polite, act very concerned, but they would still do it, because they are very, very worried about being sued. This is also an institution that will send anyone who complains about any petty thing a Target or Wawa gift card. Yes, even if you complain the blanket left lint on your black pants, or you didn't like the choice of television programming in the lobby (which, BTW, was CNN)
I am just of the mindset that jobs are hard to come by, and that before I would jeopardize someone's professional reputation I would be darn sure that there was a basis to do so.
Not to mention I have no desire to feed into, or be involved with, high school drama. Trust me, if all those kids that have dropped the OP's son as a friend, or who are already harassing him get wind of this, it will be worse. That would be especially a shame if it was all over nothing.
I would approach the mother first.