Catholic school

A father who is very much concerned about his son's bad grades in math decides to register him at a Catholic school. After his first term there, the son brings home his report card: He's getting "A"s in math.

The father is, of course, pleased, but wants to know: "Why are your math grades suddenly so good?" "You know", the son explains, "when I walked into the classroom the first day, and I saw that guy on the wall nailed to a plus sign, I knew one thing: This place means business!"