I should have gone to the conference with STBXW and when I realized (which I already knew) that the missing assignments were on her watch I should have right there said something like, "boy, you're a lousy mother. How could you let our daughter fail like that? What the hell is wrong with you. Get your act together."
It does get dicy whenever you have to tell another adult they need to do things better. I deal with it too.
You did the right thing by not speaking your mind...it's always better to take your time and say things in the most effective manner rather than saying what comes to mind first.
Me: 35|WAW: 38|D: 6yo | http://tinyurl.com/2dxx7m6 Feb 2006, left, came back in two weeks Aug 2006, left again Apr 2007, filed for divorce Dec 2007, reunited Mar 2010, moved out, filed again