My suggestion as to establishing a possible set difference between old school and new school would not be on how a particular game
could be played... but whether the rules are written to push the game to be played that way.
Sure... any particular player could have played AD&D under any of the auspices
@Remathilis posted... but that would have happened due to that player making up their own rules or ignoring other written rules in order to do so. AD&D was not written to push the game in the directions for "new school" as suggested... but that's not to say any player couldn't have turned their AD&D game into a "new school"-like experience if they made their own adjustments.
Most games we might classify as "old school" wouldn't have character personality traits that have game mechanics tied to them, for example. 'Aspects' or 'BIFTs' are more of a new school kind of thing where you get a benny to play your character's personality (for good or for bad). But there's no reason to think some random table didn't invent a similar thing for their AD&D game back in '82... the AD&D game just didn't have those rules written down themselves.