Because you knew her before your children were at the school where she works, and before she actually worked at that school. And then a load of photos appeared of her and some of other members of the department dressed up for a Eurovision party... and she was dressed in the colours of the German flag, and had drawn a Hitler moustache on her face.
Would you send her a tactful private message to suggest she didn't post that? Or would you do something else? Or nothing?
Cheers!