Reply to Kid's teacher makes mistake correcting homework

don't leave it....I know it's not exactly the same scenario, but once had a high school teacher who never graded any of our homework and just gave grades out based on her opinions of us. it let me know that unqualified teachers do exist. As the other person mentioned, they have changed the way math is taught now in many schools so it could be the case. If it turns out the teacher is really making mistakes you should report it to the principal.