Q: Did you hear about the cross-eyed teacher?
A: He couldn’t control his pupils!

Explanation: Someone whose eyes do not line up with each other, whose eyes do not look in the same direction, suffers from a medical condition called strabismus.  In that situation, people cannot make their eyes look in the same direction.  In non-medical terms, people whose eyes look in different directions may be called cross-eyed.  The term cross-eyed can be used disrespectfully or can be used as a description.  If you look down your nose, you are crossing your eyes.  In your eyes, pupils are the center part of the eye, the black circle in the middle of the eyes that lets the light in.

Pupil is also another word for student.  You might say, “After the pupils sat down at their desks, the teacher began the class.”  Pupils is not a common term in the United States; I do not know if other English speaking countries us it.

This joke is funny because the word pupil has more than one meaning: the center part of the eye, and a student in a class.

There is a band from the Philippines called Pupil and they have a song titled 20/20-