What may be more relevant for many students is whether college courses completed before high school graduation is counted for subject credit or advanced placement. For example, it can be extremely wasteful of a student’s time and tuition if s/he takes college math courses like calculus, multivariable calculus, linear algebra, etc. while in high school, but the current college does not allow them to fulfill math requirements for majors that need those courses, so s/he must retake them at the current college, even though the previously taken courses cover the same material similarly to the current college’s courses.
It does not look like UC has any limitation on granting transfer credit for college courses that were also used for high school purposes: http://admission.universityofcalifornia.edu/counselors/q-and-a/miscellaneous/#13 . Note also that students can take college courses before high school graduation or the summer immediately following June high school graduation and still be frosh for UC purposes (noted in answer to previous question in this link).
There is a 70 credit limit on the number of non-UC lower division credits that can be transferred, but all courses can be counted for subject credit even if not all credit units can transfer. See http://admission.universityofcalifornia.edu/counselors/files/Transfer_Credit_Practice.pdf and http://admission.universityofcalifornia.edu/counselors/files/uc-transfer-maximum-limitation-policy-chart.pdf .