What are some likely reasons the classroom doorknob is among the least infected areas in a classroom

Brass doorknobs exhibit an effect known as The Oligodynamic Effect.

This property of a metal means that it will react with the amine groups of enzymes or protein found in bacteria, and kill them.

Most heavy metals exhibit this antiseptic ability.

That the doorknob isn't used that recently by students, since they don't really use that to study.