I know that, but he was originally going to go home for the summer. Harry has accidentally foiled it by messing up in Azkaban and effectively forcing himself to stay in Hogwarts, but that doesn't change what Quirrell's plan was in the first place. So now Quirrell will either need to find a way to remove Voldemorte from Dumbledore's fears, or come up with a new plan altogether. I guess I should have been more clear in my comment, sorry about that.