Cal State San Marcos accepts about 70% of all applicants, so most students with decent grades and test scores get accepted. In the graph above, the green and blue dots represent accepted students. As you can see, most students who got into this member of California State University System had grades in the "B" range or higher, SAT scores (CR+M+W) of 1350 or higher, and ACT scores of 18 or higher. However, note that there are some red data points (rejected students) throughout the graph. A few students with grades and test scores on target for CSUSM did not get in.
What were the probably reasons for the rejections? Unlike the University of California System, the California State University admission process is not holistic. Except for EOP students, applicants do not need to submit letters of recommendation or an application essay, and extracurricular involvement is not part of the standard application. Thus, the reason why an applicant with adequate scores and grades would be rejected tends to come down to a couple factors such as insufficient college preparatory classes or an incomplete application.
To learn more about the California State University at San Marcos, high school GPAs, SAT scores and ACT scores, these articles can help: