There are two different ways to get certified. One is through your state, usually the State Board of Education, the same folks that certify teachers. Not all states require nor provide certification.
The other way is through the National Board for Certification of School Nurses (NBCSN) http://www.nbcsn.com/
IN Minnesota, I was certified by both the state and the National Board.
I do not get any pay differential for national certification, nor did I in my last 2 jobs. However, I think that as a health professional professional who works with educators, and we often have a educator supervisor, the certification provides evidence that we value and take pride in our knowledge base. It provides credibility and a measure of competency. "Certification assures a national standard of preparation, knowledge, and practice."