I'm going to call my teacher's pregnant wife fat
In what situation is it right to call your teacher's wife such horrible things?
She had an affair with my teacher at least 4 times
Having an affair with other people is not acceptable to you and many others. Stay out of it and let them handle it.  People are still human no matter how many affairs they have had and still can feel hurt by name calling and body shaming.
I just want to make sure that they know it's not right and they deserve to be shamed somehow now. Is that wrong?  If it is then I might stop.
Yes it is wrong as I said. People make mistakes and for that they don't have to be treated poorly. But weight and body shaming is a sore topic these days, so you are to avoid it completely regardless.