The interesting thing also isn't that Elly is too stupid to realize that the obligation never existed in the first place because she doesn't live in the real world where teachers take the overblown comments of five year olds at face value. She lives in a stupid world where, yes, damn it, Miss Lyon put her name down and will exact brutal revenge if she declines. She also lives in another, stupider world: a world in which her children will freak out and throw fits if she wants time to herself. Mike only does that sort of thing when his stupid-ass frienemies are on his case about how she's deliberately trying to embarrass his dumb ass so her concern in this area is, shall we say, overblown and silly. Mike will not die if she doesn't sit in the stands bellowing "SKATE" like an imbecile and Lizzie flat-out told her that no, she will not be crushed if Mommy just sends a plate of cookies to take to be sold.
Of course, this leads to another interesting thing: Mike's stupid-ass attempt to make up for all of this by throwing his "buddy" the Martian under the bus. Not only did he volunteer her to serve at Lilliput's, I should think that most of the reason that April had no support at all growing up is that he gave Elly license to sit on her fat ass complaining about how the princess expected too damned much.