In Liverpool, it was 'sagging school' or 'sagging off' (without the final 'G', of course; you'd take your life in your hands talking like that). I first heard the term 'bunking off' in my second or third year of secondary school - high school, as it's now called. A teacher arrived late for a lessn one afternoon, by which time, two lads had absconded via the classroom window, which looked out onto the sreet. There was, as somebody up the thread has suggested, a difference between bunking off - leaving unofficially early, and sagging - not showing up in the first place. 'Doing a bunk' is, of course, a well-known phrase for absconding. 'Barley' was the truce phrase, but was also widely used to claim immunity from being caught. It was accompanied by crossed fingers (in immitation of barley sugar?).
The knocking on the door and running away game was, in my experience, confined to 'Mischief Night' - which I think was Halloween, and I don't remember the game having a name. Not that I ever did that sort of thing, you understand.
John Kelly.