Can you please tell me how to get the mandrake root out of the box in the greenhouse please. None of the spells seem to be working, thanks.

To get into a wizard duel, simply go to the entrance courtyard and find a Slytherin. Then, stand back and target them. Even though the game says they will duel you once tageted, even if they run, hex them with Stupefy or Rictusemptra. After mutiple jinxing the will probably start a wizard encounter.

This will not work indoors. (Hence E. Courtyard suggestion.) You can also hex people indoors, but will not duel you so you can practice on them.