Cathy, you made the right choice not involving the children at all. With this being a different school K's dad probably won't have the clout he did last year. I would think about having an informal chat with DS's current teacher and thank her for helping with a smooth and successful transition to 1st for DS. And while you are chatting express your concerns about any kids excluding yours purposely.
As for K's dad, either he'll feel stupid for his past actions and avoid you so he doesn't feel even stupider OR he'll pull another stunt like that letter. In that case, I think I'd see about a restraining order against him to keep him from contacting you in any way. If nothing else, maybe that thought will make you feel better! :-)
