Everyone in my state is required to take the ACT. There are a few high schools with an average ACT score of below 14. The average doesn’t tell us how many high scorers there are in such a school, but I think it is a reasonable guess they are few and far between. And yet these schools give out just as many A’s and B’s as the schools with an average ACT of 23.
I would submit that someone with a 16 on the ACT isn’t going to be able to do rigorous college work, regardless of how high their high school grades are. That’s why the best predictor of college performance is test scores combined with test scores.