[ordinaryperson]

I'd encourage everyone to read Emily Shaw's research on this topic: https://files.eric.ed.gov/fulltext/ED563202.pdf

"Validity of the SAT for Predicting First-Year College Grade Point Average"

TL;DR: High School GPA + SAT score produce correlation coefficient values between 0.44 and 0.62, depending on the population. How strong are those values?

> A general rule of thumb for interpreting correlation coefficients is offered by Cohen (1988): a small correlation has an absolute value of approximately 0.1; a medium correlation has an absolute value of approximately 0.3; and a large correlation has an absolute value of approximately 0.5 or higher.

From Dr. Shaw's paper on page 7, linked above.

In other words it's very strong, the strongest correlation coefficient that has been found for any signal associated with first year GPA at college.

This has been well understood in the academic community for decades, it's only outside of professional education circles that the validity of the SAT and ACT (when combined with high school GPA) as a predictor of future performance at college is even questioned.