In order to begin a Credential program you must have a Preliminary Credential. For your Preliminary you first should have a BSN and a offered a position within a School District. With your first job you will receive the Preliminary Credential which is good for 5 years (actually first your District will give you a temporary one which you wait for the State to process your application). Within that 5 years you must enter a program. Some programs are all online and some are half online/half in class. The in-person classes should be in the evening because it is a graduate like program and they know you will be working while finishing the program. Some programs also offer a Master's option. Depending on the program you may have to finish a Audiometry program first or during the program. I recommend getting this done right away anyway because you need to do hearing screening in your District (actually it depends on your District, but most require the School Nurse to do screening). Programs vary between 1-2 years with some having classes in the summer in addition to Spring and Fall. After you complete the program and has worked as a School Nurse for 2 years you may apply for your Clear Credential through the State and then you're done! The Clear Credential is renewable, but the Preliminary is not. Only under vary rare circumstance may you get an extension.
Credential Program Info from CSNO: CSNO | Credentialing Information
Audiometry Info: CSNO | Audiometry Courses
Note: I did mine through CSU San Bernardino last fall. All online except for the practice exam which can be done in person at CSUSB or with a certified Audiologist that you find in your area. The class isn't very difficult. I rented the text from Amazon and there is a Calif. State School Audiometry Manual you use in the class that's free and online.