For the OP: What might help is to practice conversations in your head or on paper or something, to get used to having a response.
Seriously... write down a list of ten or 20 things they might say/have said repeatedly.
"Goodness. *My* children never had to have special tutoring."
"Are you sure you really *need* a second helping, dear?"
"Your home certainly looks... lived in."
"Oh. That was a very... interesting meal. No, FIL and I just weren't hungry."
And so on, and so on.
Now, think of at least 2 possible responses to each one.
Patiently: "MIL, remember they 'held back' learning-disabled students then, instead of helping them. Haven't you been reading the papers?" or
Archly: "I'm so glad you were fortunate that way. In *my* family we were all *above* average." (Raised eyebrow)
and so forth.
This will help you get in the habit of thinking of something to say in response. It helps, too, if you have a specific conversational gambit in mind. "Oh, MIL, have you forgotten *already* that ..... " "Oh, FIL, don't you remember? We told you that .... "
Practice, practice, practice.
Good luck! Please feel free to come back here to vent; many of us have had experience dealing with passive-aggressive relatives, and it's even harder for you to deal with without hubby's support.