Salary grid/contract

This varies district to district. In my area, only work as a Certified School Nurse counts for steps. Doesn't matter if you've been an RN for 5 years or 20 years, you only get credit for your time working in the role of a CSN. Some districts will really be jerks about it and not include time working as a CSN out of state.

This is not a nurse-only rule of thumb, they do this to teachers and anyone else considered professional staff. 

I work in MA for a public school. You should be placed on the step that matches your years of experience. I came into school nursing with 6 years of nursing experience and was put on Step 6. There is such a shortage of school nurses in MA; don't accept anything less. 

I also work in MA; I had 5 years of full time pediatric experience (inpatient and outpatient) when I began and an additional 1.5yrs of sub school nursing under my belt and was placed on Step 2 to start. The school district did not count the other work experience as relevant, but this was a tactic as at the time, they had several qualified applicants and knew I really wanted the job. I didn't even bother negotiating because I knew this is what I needed for myself. 

I work in Virginia, and it varies from district to district. My district will only take your years as a full time employee for experience in order to determine your salary step. If you worked per diem, or part time, that experience won't count.