Ok. Well one of you will have to be the bigger person in this since you both want what's best for your dad. I'm not, however, saying YOU'LL have to be the first one to be nice. See how this goes after the "initial" meeting.

oh honey, it ain't gonna be WWIII, it's gonna be WWVII. My dad married a woman i can't stand, so i feel your pain, and i wish you the best of luck

Was your hatred of each other specifically because she was your teacher? Maybe that can be put aside now? I mean it sounds like you're an adult so she shouldn't really be in an authority position over you anymore, regardless of where she stands with your dad.

honestly wish you could save a recording of this

One of you is gonna have to be the bigger person. Do you want your dad to be happy or miserable?

I'm wondering about her side of the issue. Did she grow up and move on and hold no ill will? Or is she feeling the same as you? Be the better person and welcome her to your family.

Sounds like you've already graduated high school, so why don't you solve everyone's problem by moving out?

be an adult . let it go.