Ah ok. I am not a native speaker, but would say I have a near native fluency in American English (moved here at 15 having already learned it before), and school of fish would be my go to, but shoal is the same as you said to me, sounds perfectly natural. Now that I am thinking about it though, it feels like every time I was near one (on a boat, or scuba diving), people said shoal, and in more abstract settings, school was more common. That’s probably just me inventing a pattern though