Lizzie provides us with a clue as to why this is when we discover that despite her teacher's belief otherwise, not all children can actually do mental arithmetic. It would seem that the two older children have inherited not only looking like members of the Richards family from Elly, they've also, as I've said, inherited her inability to understand math all that well. They've also inherited her belief that doing something that bores them and is difficult is a bad thing to do because by doing so, they're being deprived of happiness by mean people who hate them.
The reason that I mention this is that I really don't think that Phil and Elly's emphasis on music theory and notation was all that helpful a way of teaching Michael the trumpet. This is because just as mathematical notation is simply baffling gibberish that his evil teachers use to inflict misery on him so they can laugh at him for being stupid, Michael's brain simply rebels in horrified rage when confronted with musical notation.
That being said, it would be fairly easy to make a success of teaching Michael to do basic arithmetic. First, you'd have to appeal to his monstrous vanity and tell him that he can finally beat all the mean teachers who wanted to trick him, humiliate him and take away all his fun because teachers have no sense of human. Next would come a means of teaching remedial math to people like him and Lizzie; once Mike finally understood that fifty percent isn't evil gibberish that was designed for the sole purpose of mocking him, he'd finally realize that his real worst enemy was never his teachers or Sistwirp or Brad or Mira; he has to look at his worst enemy in the world in the mirror every day when he shaves or brushes his teeth.
Oh, wait. He never wants to see that he's always been his own worst enemy. Better to do what John planned and have Anthony become his math guy.