NISS is a bottom feeder third rate internet recruiting agency. They simply plug teacher candidates into position at bottom tier schools. They are the recruiting equivalent of an ESL school putting "International" in their school name (and many of NISSs client are exactly those kind of ESL or Bilingual schools. They are no different then any number of third rate recruiting agencies such as TeachAbroad, IEG, and the various other general job sites that happen to have an education/teaching category.
KAK Maybe you don't know the answer to this question, but if the schools are not connected, do they follow the same salary schedule and benefits package? Did you get a position at your school through the school itself or did you interview with Nacel first?