"And a letter signed by your guardian."
Harry's world crashed down - he knew how impossible that was, and how unfair, and clearly Snape did too. The man was practically smiling. "But - I haven't spoken to them all year! They're Muggles; they don't even take owls!"
"That is the rule, Potter, and it is one you will follow." Harry usually had good reasons for breaking rules. Snape even knew about some of them, at least. Maybe he wasn't a dark wizard, but he'd given Harry more than enough reason to hate him.
"It's not as though they even want to see me!"
"And no doubt Lucius Malfoy has more important business than minding an insolent brat all summer." If the professor's expression darkened, Harry didn't catch it. Draco was worse than him, but Snape never noticed that.
"He said I could come!"
"Nevertheless, Potter, you will not be going."
"Isn't that for Aunt Petunia to decide?" Harry muttered, before recalling his coaching. He took a breath, and went on, desperation bringing forth candor. "Professor Snape, all they're going to do is lock me in a cupboard for three months - why shouldn't I go with Draco?"
"Because I have seen nothing in your attitude or conduct to suggest that you deserve it. Two hours' detention for your insubordination, Potter. Get out."
Harry slammed the door after him.