Defensive people attack
Vulnerability leads to violation: the best defense is a good offense. Once Mrs. Smith launches her tirade against you, you feel vulnerable, and so you up the ante: “Maybe you don't understand me, Mrs. Smith. If Jimmy doesn't start doing his homework, he's flunking.” So now Mrs. Smith feels more vulnerable, and so she ups her attack and threatens to go over your head.
You'll notice the pattern doesn't read Vulnerability —> Violation —> Homework. Homework isn't on this chart. Neither is any other sort of progress.
Sometimes the same pattern can play without including you. You tell Mrs. Smith about Jimmy's homework. She promptly reaches over and smacks the kid and tells him he's worthless, just like his father. She feels vulnerable and so she violates Jimmy.
Check the chart— belting Jimmy won't lead to homework, either. Jimmy feels vulnerable and disgraced, so he'll search for a way to even the score. He might grudgingly limp through some homework for a while, then act out in some other way like punching the smart kid in class. The cycle is still in motion.
Head off defensiveness
The best solution is to head off this pattern before it starts. When Mrs. Smith walks in the room you do not want her feeling vulnerable. Welcome her at the door. Sincerely thank her for coming. Have some food or cookies at the conference table. Food is a universal peace offering, a way of smoothing tension. Friends and family eat together. Enemies do not. Feed her.
At some point early on, find some reason to tell her she's a good mother. OK, it might be a stretch, but there's an observation made by Winston Churchill. He found the fastest way to get someone to acquire a quality was to ascribe it to him. Perhaps she's a good mother because she loves her son; perhaps because she came to the conference. Certainly millions of good mothers love their kids and can't figure out how to get them to do homework. Just the assurance that she is not alone can help calm her down and win cooperation.
Watch your language
Even if you make inroads with Mrs. Smith, the vulnerability is still lurking, so it's important to be tactful and respectful. When you make your suggestions for Jimmy's homework, avoid the word ‘Should,' as in, “You should set aside a time every evening,” or “You should turn off the TV while he's doing homework.”
“You should…” translates to “You're stupid,” and that's going to get you another argument. It's not going to get the TV turned off.
You both want good results for Jimmy, you just need to know how to get there. Mrs. Smith may not be quite such a bear if you can avoid triggering her fears.