I remember that very clearly. I think I experienced the only group of PE teachers who weren't using the plimsoll regularly, one of our music teachers was a different matter though, don't think he could get through a music lesson without slippering someone.
Yes it is very effective. Six strokes nude completely nude, EVERYTHING and I mean EVERYTHING off, is much worse than six strokes bare trousers and underpants down given at the same strength.
I always found PE teachers to be obsessed with the subject of underpants, or more precisely being obsessed with boys removing their underpants for PE and being bare under their PE shorts.