Well, and I for one think once you've reached your majority, you should be allowed to leave campus whenever you feel like it. I mean really, what's to prevent Ron and Hermione sneaking off to Hogsmeade and Apparating? Or using the floo connection from the common room? I don't intend to strand Sirius at Hogwarts, at the tender age of sixteen, without any way to talk to the one really good friend he's made since he arrived in 1997...:D