What makes a story fit in the NiS universe?
I have no connection whatever to anyone able to make that decision, but as a frequent reader of NiS stories, I'd like to identify some characteristics and invite others to also list and expand on other characteristics if they choose.
First only a few students at a time are chosen to experience nudity, normally for a week, although sometimes their week gets extended and a few get selected based on their behavior for a day, or sometimes more as a discipline issue. There is a phrase, "We are all in the NiS program, only a few are actually naked." The program (or Program) includes "reasonable requests" that need to be accepted by naked students, mostly from other students between classes or sometimes at lunch or after a few minutes given the selected to actually eat something. Teachers can make requests that may or may not be reasonable and usually involve some sexual situations. The naked participants come to school, most often dressed and have to disrobe and store their garments before entering the school, after the first day, and can put them on after leaving school at the end of the day. There are stories that encourage "outreach" so that nudity continues after school. The week runs from Monday morning through the following Sunday, and school activities after school also require nudity, perhaps a school play, athletic event, musical performance or almost any other school program (perhaps a SAT test?) whether on school premises or otherwise. There is usually a period, often the first five minutes of class where masturbation, sometimes mutual or full sex are permitted as constant arousal is "hard" to live with. Sometimes only boys are allowed this period. Use of the opposite sex bathrooms and locker rooms and often gym classes are required. Showers with one male and lots of females or one female and lots of males are encouraged or required. Many of the stories have partners of opposite sex assigned to help protect the girls against rape or non-consensual activity. Reasonable requests need to be made and accepted before touching is allowed, in most stories failure to get consent makes any subsequent activity wrong and sanctionable, the decision on what is "reasonable" sometimes is a key to the story. Sometimes romantic attraction occurs between partners, but it is not part of the NiS cannon.
There are discussions on the ASSTR NiS site that go into more detail and discuss the positive (NiS is good) and negative (NiS hurt participants) story types. If anyone wants to write in this universe I will be glad to read their story. You might want to read a few before beginning a new story in the universe. As indicated in earlier postings you may get feedback, not entirely positive, if you decide the Program in your story differs significantly from the ones other stories describe. Strangely, Karen Wagner's original story does not conform in every respect to the current idea of what the Program is like.