That's because the Patterson children are cursed with the wretched study habits of their parents and the specious logic that makes them throw temper fits about being tortured with impossible questions such as "If having 5 apples means that you have twenty-five percent of the apples, how many would you have if you were given eighty percent of them?" (Answer: sixteen.) Someone with good study habits and the belief that mathematics is his or her friend would be able to work it out sooner or later; since the Patterson children have it in their heads that their teachers are trying to embarrass them by publicly exposing them to ridicule and since they hate being told what to do and since anything that takes a bit of effort is eeeeeeeeeeeeevil, we get a screaming match.
What John and Elly refuse to admit is that this doesn't happen in a vacuum; granted, Elly does seem to be intermittently aware that her children's behavior is somehow shaped by the way she lives her life but she tries to deny it so as to live in her preferred state of constant confusion. In this, she is aided and abetted by a husband who has no conflict because he can't think; the same John who twists every fact to make sure that the real world matches the false one in his head won't admit that his defects as a student affect the odd little people that have the baffling and evil habit of pretending that they're not extensions of his will. He can't make the connection between his struggling for every passing mark he ever got because he came in to things with the attitude that his teachers were trying to trick him into admitting that what everyone knows is something that no one does with his own children's stubborn refusal to be taught.