"Math at a Catholic School" joke

Little Tommy's parents had tried everything to help his math grade:
tutors, flash cards, "Hooked on Math," special learning centers,
everything. Finally, they enrolled him in the local Catholic school.
The very first day, he came home with a very serious look on his face,
went straight to his room, and started studying. His mother was
amazed. Books and paper were spread out everywhere and Little Tommy
was hard at work. As soon as dinner was through, he marched right back
up to his room without a word and studied some more.
This went on for weeks until Little Tommy proudly brought home his
report card and showed it to his parents: An A in Math!
"Tommy! This is great! I'm so proud of you! Son, what was it? What
helped motivate you? Was it the nuns?"
Little Tommy shook his head.
"Well, then, was it the books? The discipline? The structure? The
uniforms? What?"
Little Tommy looked at her and said, "Well, Mom, it's like this. When
I saw that guy out in the lobby nailed to a plus sign, I knew they
weren't screwing around!"