Why can't I access the key during the first Defence Against the Dark Arts class?

I am stuck in the Defence Against the Dark Arts classroom in LEGO Harry Potter: Years 5-7. I have unlocked the potion and collected everything, but the sparkly arrow is pointing to the locked door, and when I send the pet up the tube and walk along the ledge, the key I need is not knocked down.

Can anyone help me get the key?

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