ACT score is correlated with many factors, which can make it difficult to draw such conclusions. For example, what if a student has a high HS GPA with a rigorous schedule and a high SES, does he still have a low grad rate with a 22 ACT, or does his grad rate more follow his GPA and SES than his ACT score?
The study at http://www.heri.ucla.edu/DARCU/CompletingCollege2011.pdf looked at the relative contribution of test score, HS GPA, SES, and many other factors for over 200,000 college students at hundreds of institutions . They found that with a full model that includes academic characteristics, SES/financial characteristics, major/college characteristics, and time spent on various activities during HS they could explain 26.9% of variance in 6-year grad rate with 71.4% successful predictions. When they excluded test scores from the model, the prediction dropped by only 0.1% from explaining 26.9% of variance to explaining 26.8% of variance, with both successfully predicting graduating for 71.4% of students… essentially no difference. Some of the stronger positive predictors of graduation in 6 years were college college cost (higher is better), self rating of drive to achieve, and experience using Internet for HS assignments. Some of the stronger negative predictors of graduation were planning to live off campus (instead of dorm), planning to transfer, and planning to work full time while in college.