Here is the full list of all 6 Absent achievements. It takes around 4-5 hours to unlock all of the achievements on Windows.
Offered Curly Fries to all five available characters.
Completed Day One.
Read your homework to all seven available characters before handing it in.
Unlocked the next location in Murray's journey.
Murray solves the puzzle and opens the cave.
Congratulations, you have completed the game!