I don’t know what numbers look like overall, but I do happen to know that the CIF athletic association (Cate competes in the southern section) only allows for 8 consecutive semesters of eligibility once a student begins 9th grade for the first time. There are a few loopholes but they require injuries or other major events that require withdrawing from school for an entire semester.
On the other hand, NEPSAC’s eligibility rule is age based… the athlete becomes ineligible the September 1st after they turn 19. That allows a lot of students a fifth year.