Here is the full list of all 10 Harthorn achievements.
Finished your first morning shift at Harthorn High School
Finished your first evening shift at Harthorn High School
Find the Harthorn Key in the abandoned school bus
Crawl across the air vents to continue your investigation
Wake up in “The Dungeon” of Harthorn
Find the mysterious mask and Tarot card
Turn all the Valves in the Condemned West Wing
Disarm the explosives in the Condemned East Wing
Defeat Victor in the your last encounter
Survive your time in Harthorn High School