On my YouTube channel I've created gameplay walkthrough videos covering this game.
In short, for those who want to avoid all spoilers, this unlocks after you complete your first class and talk to the student you meet in the class. The achievement is unlocked at about 35:20 in the video.
For those who wish to know everything you have to do before unlocking this:
*** Spoiler - click to reveal ***After completing the opening of the game and the Sorting Ceremony Professor Weasley will lead you to the dormitory for your house. When you wake up the next morning find your way back out of the dormitory to meet Professor Weasley again. She'll give you a Wizard's Field Guide. Complete another set of tutorials ending with Professor Fig and you'll be turned loose to explore the castle with 2 main missions, to attend Defense Against the Dark Arts and Charms classes. You can choose whichever you like for your first class. Just open the map, pick the one you want, and select it. The mini-map will guide you to your chosen destination. The classes are simple and will each teach you a new spell. Once the lesson is over you will have to talk to a student, Sebastian Sallow for Defense Against the Dark Arts or Natsai Onai for Charms. After the conversation your achievement should unlock.
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