Son - Dad, I've been expelled from school for having sex with a girl in my class.
Dad - Son, that's the 2nd school this year! Maybe teaching isn't for you!
Son - Dad, I've been expelled from school for having sex with a girl in my class.
Dad - Son, that's the 2nd school this year! Maybe teaching isn't for you!
The humor comes from the dad's unexpected reaction. Instead of focusing on the son's misbehavior, he interprets it as a sign that teaching might not be the son's calling, implying he's the teacher in the situation, and thus that this is not the first time the son has been expelled for this reason.