The "a-g" courses also give you the skills to be prepared for college classes, so even if not required (for community college admissions, for instance) these classes will still help you advance.

For most 4 year college admissions, you will need to pass these classes with a C- or better. For competitive colleges/universities, you will want to complete not just the required but also the recommended "a-g" classes and get all A’s and B’s.

Free Online A-G Classess
Not every highschool in California offers the full A-G curriculum that a student needs to complete to be elegible for the CSU or UC admissions requirements.  UCScout.org offers these high school classes online for students to complete. Read more about it here