Exit Pupil

No, the exit pupil has nothing to do with getting expelled from grammar school. The exit pupil is the circle of light a shooter sees when the scope is held at arms length. It is a measure of the quantity of light that reaches the shooter's eye, and is not influenced by the diameter of a scope's main tube.

In typical low-light conditions in the field, a shooter's eye dilates to a pupil width of about 5 mm. If the exit pupil of the scope is smaller than the shooter's pupil, too little light will reach the shooter's eye and the scope will impose limitations on the shooter's ability to see.
The diameter of the shaft of light exiting the scope toward the eye is used to rate the brightness of a scope's sight picture. For example, a 4X scope with a 40mm objective lens has a exit pupil of 10mm. The larger the exit pupil, the easier it is to keep the eye aligned with the sight picture and the better the scope will perform in low light.