I honestly think both the NP and the CNS are good preparation for the school nurse role. I lean toward the CNS (it is the choice I made) as the NP gives a bit more emphasis to the primary care provider aspect of school nursing and about half of the job is health prevention and promotion. Having said that, I know many FNPs and PNPs who are school nurses and are great and you will definitely use your high level assessment skills. If you would prefer to have the option of prescriptive authority and work in a clinic setting, now or later, then go for the NP. Unless working in a School Based Health Clinic, you would not likely get to prescribe at school, but I do know of a few exceptions to that, school nurses who can prescribe - they are usually in private and or boarding schools. It is definitely an advantage to be able to perform the school physicals.
New Jersey has a required school nurse certification that includes some masters classes. You would likely need to complete that before you were able to get a job. I am not familiar enough with NJ requirements to know if you can work on it after you are in the position. The NJSNA website has the certification requirements and a list of all the schools that have the certification programs: http://www.njssna.org/
If the school nurse position is on the teacher's salary scale, and if the teachers get tuition reimbursement, then, yes, it is likley that your master's degree will be reimbursible. But ask to what extent - it may not be as generous as in the hospital setting. It is my understanding that most school nurses in NJ are on teacher's salary scale - but ask. Usually in non teacher salary scale positions the pay is good to start, equivalent to what nurses are getting in other districts, but it makes a big difference in the long run with salary increases, benefits and retirement. Just like the teachers, the pay scale takes into account the number of credits at the masters level.
NJSNA is having a conference in September. http://www.njssna.org/pedconf.htm
If you are able to, go and use the opportunity to ask questions. Bring your resume. I attended a state level conference after relocating to a state, met a school nurse adminsitrator with an opening 15 minutes from my home, was interviewed on site and started two weeks later.