PC - I missed this yesterday...how truly truly awful! The fact that she is a parent at the school. That's all you need!
His behaviour really is outrageous. He has no business swanning around being treated like royalty by your Mum.
How did you find out who the mystery tart is?
But...as you say, you are way way too good for him, she has got a man who is not even learning from his mistakes. He's hardly heading for the kind of relationship that would make you think 'why couldn't he have that kind of relationship with me?' - which would have been very poignant, after all the hard work you put intring everything possible to make your marriage work. But no, he has proved that he was not worth sticking around for, so hold your head high, and feel sorry for the poor woman who has to pick up your cast-off. The woman can't be stupid enough to see that you are totally gorgeous, well-thought of, good at your job...so she will know that it's your exH's loss and that he would have kept you had he had the choice.
Your life, PC - you couldn't make it up!
And as for your parents - no don't tell them, they will probably invite barwoman tart to the golf club. (oops, was that way too sick??? ) But as you say, however they react won't be of any help or support for you.
Play it cool..you're doing very well!