In general, as far as I know (and I am not an attorney or have otherwise any special training in this legal matter) so long as the model isn't engaged in a sexually explicit manner then it is protected. However, what "sexually explicit" entails is so highly variable from person to person, religion to religion, culture to culture, it's almost impossible to know if you've crossed that line. I believe in one case, Mann perhaps, the judge concluded that pornography was something "he knew when he saw it".
Some people who are very conservative would say any form of nudity is sexual by it's nature, and society as a whole has a "better to lock up a person for child porn than risk a pedophile on the street" kind of attitude. It's just not worth even approaching, IMO, unless you are a making specific statement which cannot be done any other way - then you need to ask yourself how dedicated you are to this project, because it is very possible that it will blow up into a huge deal. If you've just found a pretty, 16 year old model whom you want to photograph nude, OTOH, forget it.