WARNING! I found a dead end bug in Level 12.
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I had completed all the tasks in the classroom memory, but Oops kept telling me he couldn't wake up the bad guy from hypnosis, because there was "something he forgot". I really couldn't do anything more, so I watched a walkthrough on YouTube . I discovered the ending of the level is triggered only when you help little Morglot AFTER you help his classmates! In my playthrough I had completed all the task to help little Morglot BEFORE I helped all his classmates. The game seems to expect a precise order in the actions, but the problem is you can still solve the classroom puzzles in any order! I don't know how this elusive bug could be fixed. Pierre, I could suggest you to make sure the level ending is triggered whatever order the player follows to solve the classroom puzzles! :-P