I did some research. Keep in mind, I’m a moron, so I may have screwed up somewhere along the way.
Orelvis Martinez currently has a .306 iso and a 26.1 K% in 280 plate appearances as a 19 year old in A ball. I wanted to see how rare it is for a teenager to put up a .300 iso in a single season in full season ball. I used 240 plate appearances as the cut off (for some reason), and fangraphs only goes back to 2006. I got:
Giancarlo Stanton, 2008. .318 iso; 28.3 K%
Joey Gallo, 2013. .365 iso; 37.0 K%
Seuly Matias, 2018 .320 iso; 34.8 K%