Psychonauts/Basic Braining
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At the start, the class is rushing out towards Coach Oliander's Basic Braining course. While you can speak with the characters in the surrounding area, there is not much room for exploration.
Head up the spiral staircase. You will receive Smelling Salts once you reach the top; this forces you outside of the Psychic World (with one exception) landing you outside.
Once you are ready, head inside the treehouse and speak with the Coach. When you are ready, he will open a psychic door and bring you inside his mind.