Personally, I'd get the divorce before having the date because you don't want to lead someone on and then decide you want your comfortable roommate for life. That said, maybe your wife really meant you could have an affair and she wouldn't mind. If high school lady is cool with it, then that's an option. It's my humble opinion that dating while divorcing really fucks with your emotions. I speak from experience on both sides (the divorcing person and the person dating a divorcing person).
Either way, good luck.
Either way, good luck.