I work in a school as Drug preventionist, so I am not a school nurse (but I am a RN, MSN). I formerly worked in a county health department where we provided some services for schools
that didn't have school nurses.
What exactly happens varies widely from school to school (district to district) but in general nurses are in on sexuality education from the information point of view and in the US, communities are in to local control of education (at least in KS) and so our state department of education makes recommendations but actual curriculum is a local matter, though our state board of ed did mandate AIDS prevention education. I've been in schools where they wanted me to show birth control information to middle school students, high school students AND at the other end of the spectrum, where they didn't want us to talk to high school students about condoms. Local norms vary. SOme US schools will have affiliation with school based clinics, some of which will be on-site. These usually have some kind of birth control protocols. Morning after contraception is touchy. I would gauge that some schools have this protocol and some do not.
In general (for both birth control and substance abuse) the important thing to do is to create advisory boards made up of you, teachers, parents, administrative reps, educate them about the issues and then get their support in developing policies. Usually useful to know what other school districts around you are doing. Any policy or curricula in my school is subject to approval processes. Always have your community informed about your policies, that way the policies grow out of community norms and even if you don't get all you want at first, sometimes you can show your community that you will do what you do intelligently and sensitively and that they can trust you and maybe they will broaden your policy to be where you want.
Even if your community won't support your provision of morning after contraception, know where you can refer students for this service. School nurses I know were unable to transport students to contraceptive appointments; they simply could not take on the accountability to transport a student without parental knowledge to a medical appointment. Know the rules governing your interactions with students.
I would look to take field trips to programs that run clinics like you hope to run.