In high school we had college teachers who taught some of the courses. They were willing to teach it at a higher level so that we could get college credits if we were willing to enroll and pay tuition.
I earned many credits this way but I don’t think it was worth it financially in the long run.
(Some of the courses weren’t necessary for the degree I was seeking. The math courses were all prerequisites for Calculus I, which would have all been satisfied anyway just by passing Calculus I. Yeshiva credits filled up the remaining credits needed to reach 120.
Had I not been in yeshiva or received yeshiva credits those courses would have been very helpful to satisfy electives.)
Theoretically I could have forgotten about graduating from high school since I had enough credits to apply to college as a transfer student (and they don’t need to see a diploma) but my dear mother couldn’t bear the thought of her son being a high school dropout.