Given that the only thing Harry seemed to manage to keep a secret from Quirrel is what he really transfigured Hermione into, I suspect Harry might have to win by using the trick he did with the Troll, but with Hermione instead of a rock.
Crazy hypothesis: he transfigured Hermione’s body into a copy of himself, and sent the copy in his stead.
Given that the only thing Harry seemed to manage to keep a secret from Quirrel is what he really transfigured Hermione into, I suspect Harry might have to win by using the trick he did with the Troll, but with Hermione instead of a rock.
Crazy hypothesis: he transfigured Hermione’s body into a copy of himself, and sent the copy in his stead.