Fantasy. Magic world, it’s a school. In the middle of it they go to a cabin to do something and one of them turns into a ferret fighting someone. 400-500 pages. Read 2012.

[Wendy]

Harry potter had person turning into a ferret. Got. But I also think
Diane Duane Wrote a book that had that in it also. Might have been a mink.

[inaword]

The fourth Harry Potter book specifically

[Florence]

Long shot but maybe wilder girls by Rory power?

[Rainbowheart]

Wilder Girls for Florence's suggestion.

[Kris]

Camila, is this book for kids or teens? How old are the main characters?

Can you describe the magic world (and how it's different from the real world)?

[Kris]

A long shot - The Novice by Taran Matharu?

Some Goodreads reviewers say:
- "Fletcher summons a ferret like demon, which takes him on the run to learn more about his new powers and help in the war against the orcs."
- "Ignatius was my favourite! He was just so cute, I imagine like a miniature Toothless. He's like a ferret/dog/dragon, obviously the best combination."

[Lobstergirl, au gratin]

OP's account deleted. Moving to NWNK.