UC Santa Cruz rejects about a third of all applicants, making it a moderately selective member of the University of California. On the graph above, the blue and green dots represent accepted students. As the data shows, the majority of students who got into UC Santa Cruz had at least a B+ average, an SAT score (CR+M+W) above 1500, and an ACT composite score of 21 or higher. Chances of admission improve as those numbers go up. In the center part of the graph, note that there's some red hidden beneath the blue and green. Having grades and test scores that are on target for UC Santa Cruz is not a guarantee of admission, especially if some application components don't compare favorably to the rest of the applicant pool.
UCSC, like all of the University of California schools, has holistic admissions, so the admissions officers are evaluating students based on more than numerical data. Students who show some kind of special talent or have a compelling story to tell will often get a close look even if grades and test scores aren't up to the ideal. Impressive extracurricular activities and strong essays are all important parts of a successful application to UCSC.
To learn more about UC Santa Cruz, high school GPAs, SAT scores and ACT scores, these articles can help: