I grew up in Scotland in the 1950s and 60s and like a lot of Scottish boys, I always wore bottle green school knickers under my kilt. They were Cherub bloomer-type which were upgraded (eventually) to the briefer type with cuff legs in the 60s.
I wore the kilt every day, not just on Sundays. In fact, I wore the kilt to school up to the time of leaving. I even had a ‘scout kilt’ for a while, which was of heavier material. My mother used to refer to the knickers as kilt knickers, as indeed did a few of my pals who were similarly attired.