OP, I went straight into school nursing 1 year ago right after I passed the NCLEX and haven't looked back. I love it! However, being certified or credentialed can depend on the school and really, the best foot into the door is subbing. More schools
, even public ones, don't require certification to sub, but again, it can differ widely between states and districts.
I work at a public charter school and I am working on my school nursing licensure, but not having it did not stop me from getting the job. However, a public school in my neck of the woods (MA), they want the licensure first, which requires a BSN and 2 years as a school, pediatric, or community nurse (and a test/coursework, etc). Private schools usually do their own thing, but again it can vary.
The CA School Nurses Organization should offer some more insight: CSNO