Link to California Family Code 6924 (and the 6900's in general which deal with consent to services).
UCLA School Mental Health Project Page with links and resources to the practical concerns, including consent and confidentiality, of providing mental health services in schools.
Webpage with a ton of summaries and links to resources on the topic of informed consent. (Scholarly and national level, not CA specific).
National Center for Youth Law's "Minor Consent, Confidentiality, and Child Abuse Reporting in California"
National Center for Youth Law's Summary of Minor Consent Law's for CA. (A great index to reading the law directly.)
Disclaimer: Some of these reports may be a little dated if legal changes occurred recently. However, the legal citations should allow you to cross-reference to the direct legislative text.
Specific California laws can be accessed for free at http://www.leginfo.ca.gov/
Also note that I have not yet found a great set of resources that definitively and concretely answer the question, "At what point does seeing a school psychologist and/counselor in the schools meet the threshold of mental health services and, therefore, requiring formal consent." If you have any good ones, please post!