I think it varies from district to district. In PA, you are usually on the same pay scale as a teacher (if you are certified). I am starting next week at around 45,500 (no prior school nurse experience but they counted my 3 years of hospital experience). Non-certified nurses make much less (some as low as 20,000-25,000).
You're by and large always going to be on the teacher pay scale - private schools may differ, but most don't and as far as starting salary, it depends on the district and what step they're starting you on. Some may take into consideration if you have experience in a hospital and may choose to bump you a step or two, but many do not and don't advance steps for anything but educational experience. Starting salaries can range depending on the district (meaning step 1 with BA) any where from 35 k to 50 k in my part of the garden state from what i've seen