Unexpected lesson

Harry was eating breakfast when Hedwig swooped down and dropped a letter in front of him. He looked over at Ron who also had a letter and exchanged quizzical looks. Harry opened his letter, it simply said: Meet me in the Transfiguration class tomorrow night at 8pm. H.G.
"Who’s H.G.?" Harry asked Ron
"no idea" replied Ron "should we go"
"guess so."