Is this starting immediately?
Take it.
That is a big school, but with the health assistant, it does put you in the range of the recommended ratio of 750:1. I'd ask about the percent of special education and your role in contributing to the IEPs, how many kids in the building have IEPs and how many medically fragile and chroniclly ill children there are in that group. Ask specifically about tube feedings, catheterizations, vents or trachs, diabetics, kids with seizures, CF needing chest percussion / vest treatments and asthmatics needing nebulizing treatments. It is possible that the school would have none of the above, but also possible that it could have quite a few. If you do not know what an IEP is, don't worry, most should have been done by this time of the year and you can get up to speed before the fall.
You will need to delegate the "traffic" in the office to the health assistant, so you can spend time on planning the screenings and completing care plans. Right from the start do not get bogged down in the skinned knees and paper cuts. You will want to track the number of bandaid type kids who are being sent right from the start and institute a program where the teachers get basic bandaid supplies and guidelines on who not to send to the school nurse. Also try to delegate the immunization tracking in the fall to the health assitant and you would get involved only with those families that need some prodding or assistance.
Ask for a mentor and orientation in the district. If starting immediately, ask to spend one day with the mentor and then for a sub experienced in that school to be with you for the first 2 - 3 days.
Contact your state organization and ask about summer conferences and state orientation. Join. It looks like you are in Arizona and they have a strong state organization.
http://www.snoa.org/ The SNOA Summer Conference, JULY 17, 18, 19, 2006 You could even ask them if they have a mentor program. Some of the state organizations do.
Good luck!!!!!!! Of course there is a learning curve at first, but it sounds like you are ready for a change.