A Rebbe of mine related a story from when he was in high school.
There was a classmate who was a big massmid but had zero respect for secular studies. The Rosh Hayeshiva called him in to see what was going on.
The bochur explained that it was bitul zman since he had no use for a high school diploma anyway.
The Rosh Hayeshiva explained that that’s where he was wrong. “It’s bitul zman not to take secular studies seriously since you have to be in class anyway- you’re wasting time by not accomplishing anything.”