Almost anywhere where someone won't be deliberately looking for a transfigured item or where they won't accidentally pick it up. You can't search the school in general for transfigured items, it would take too long. He could simply put transfigured corpsemione into any random book in the library or hand the item to someone to hold for him for a while or put her under someone's bed or anywhere as long as he can plausible retrieve her before his transfiguration wears off.