This achievement is earned for finding optional photo #6. If missed on the first run, you can "Play Episodes" and select specific parts of the Episode in order to find the missing optional photos through collectibles mode. The following collectible is described and can be seen below:
] - Optional Photo #6
Back in the classroom, interact with the etched markings on the desk in the middle of the room. After interacting with it, take a photo.
Collecting all 10 photos will also reward you with the following achievement: