I'm guessing dot product comes into play in the expected answer. When the short person pushes, his push is more straight and less down and therefore more of it is useful. The dot product of two vectors show how much of the first vector is in the direction of the second vector.
Dot product - Wikipedia, the free encyclopedia
The second vector will be the unit vector parallel to the floor in both cases. Assume both people push with a force equal to 1 unit. The dot product for the tall person is
and the dot product for the short person is
. We'll assume that the box is shorter than both people's shoulders.
will be in the range
and
so cos in that range will always be less for the tall person and therefore their dot product will always be less. If you want to add friction you can show that the dot product of the tall person's vector with the unit vector pointing down will be greater and therefore the force of friction will be greater.
Does that sound like something more suitable to your math class?