According to Jim Recap: ABCs and 123s
When Ruby starts having problems with her homework, Jim suggests that perhaps he could help out. But Cheryl being Cheryl, that isn't likely to happen, at least not without a fight. So when Jim starts to help and rapidly finds that Ruby isn't doing any better, he goes to Ruby's teacher to see what can be done about it. When he finds out that an exception can be made for a problem at home, he decides to come up with a reason to lighten Ruby's load.
The only problem is that perhaps the reason he chooses isn't the best one. While Jim is undoubtedly good at coming up with a scheme to get out of whatever he is trying to do, sometimes he has to do so quickly, and so looking around the room for some inspiration, his eyes light on a reading poster. That in turn leads him to tell the teacher that Cheryl can't read.
This does achieve the desired result - it gets Ruby less homework - but it means that the teacher starts sending notes home with Ruby, addressed to Jim, and soon Cheryl gets suspicious about why she isn't receiving these notes and why they are going to Jim. Confronting the teacher, she gets the whole story. That is when she sets her plan into action.
The next PTA meeting comes to their house, and when Jim is confronted by one of the other members about Cheryl's "problem", he knows he'll have to attend so that she doesn't found out what he did. Before long, he's volunteered for a number of projects, and just as it looks like he's about to bust, he has to read the agenda, where it says "don't try to put one over on your wife". Nicely done.
