The following link, to a column published by the American Psychological Association called "Ethics Rounds," deals with exactly the issues about which you are inquiring, and may be helpful.
Not legal advice, just general information. I hope it's helpful. I don't practice law in California or hold licensure there. If you need legal advice consult California counsel. I practice in Vermont ONLY.
First, a school nurse may not be a therapist, so telling a nurse something personal and confidential may not be a really good idea. Licensed therapists or doctors are not allowed to disclose confidential matters unless the matter involves something that they are required by law to report. For example, if a patient tells her therapist that she is abusing children, a therapist has a legal obligation to report that to the proper authorities. A therapist is often required to report to authorities if he has a reasonable belief or fear that the patient is engaging in activity that could put the patient or someone else at risk of harm or death.