De nada, Angie. (You're welcome, Angie)
Bueno, creo que la questi�n es que los alumnos se r�en de �l. (Well, I think the question is the students laugh at him.)
Pero primero parece la t�pica respuesta de un ni�o que no quiere ir a la escuela. Y luego sabemos que �l es el director de la escuela. (But it first seems the typical answer from a boy who doesn't want to go to school. And then, we know he's the principal of the school).
The question is that the the first part of the joke looks like a boy who doesn't want to go to school, with all those excuses. And then his mother tells him he has to go, because it is his obligation, second, because he's 45 and third because his the principal.
When he says they laugh at him, it could first be understood as boys laubh at him at school, but when we already know he's in fact the principal, seems boys keep on laughing at him. :-)