Defender of the Faith

 is a secondary quest in The WItcher 3: Wild Hunt where Geralt encounters an old lady requiring help fixing three shrines.

Walkthrough
Geralt can encounter this woman between Blackbough and Crow's Perch (due west where horse races in Crow's Perch starts). Accepting her plea, Geralt fixes the shrine. However he must fix two more shrines. Once Geralt fixes the second shrine, he must now turn to the last shrine. Upon arrival, Geralt encounters the vandals destroying the shrine. Letting them do what they pleases fails the quest but completes it by beating them.

Note: Regardless of the order of what shrine Geralt fixes, the students would appear at the last shrine Geralt visits.