First Corps to sit down on the starting line I believe was, St Kevins at the Eastern Mass Finals. The inspection judge had gone through (5) people and already had marked the Corps off by 1/2 point so the DM, Walter Curtis cut the inspection short and ordered the Corps to "Sit".
I believe this was the first and then the Reveries did it in Roosevelt Stadium in later years. (MM instructor, Walter Curtis) Hmmmm