Are ACT Scores Correlated With High School GPA?

Do students with higher high school GPAs get higher ACT scores than those with lower GPAs? I have a 4.0, but I didn't feel as confident with the ACT Saturday as I generally do with my grades in school.

Yes, there is a correlation. This research article published by ACT, Inc. goes into greater statistical detail, but in addition to there being a correlation between high school grades and college success and ACT scores and college success, there's also a correlation between ACT scores and high school grades. The higher your high school GPA, the higher your ACT score is likely to be (and vice versa). Correlation does not imply causation, though, so doing well in school will not necessarily automatically cause you to do better on the ACT.