There are 9 schools, or ryuha to give them their Japanese name that are considered Ninja schools
1. TOGAKURE RYU NINPO
2. GYOKKO RYU KOSHIJUTSU
3. KOTO RYU KOPPOJUTSU
4. SHINDEN FUDO RYU DAKENTAIJUTSU
5. KUKI SHINDEN RYU HAPPO HIKEN
6. TAKAGI YOSHIN RYU JUTAIJUTSU
7. KUMOGAKURE RYU NINJUTSU
8. GYOKIJSHIN RYU NINPO
9. GIKAN RYU KOPPOJUTSU
only 3 of these are actually "ninjutsu" or ninpo in the truest sense
the other 6 were MAs practiced by Ninja clan that became complete schools - some are based on Kempo, some of Japanese JJ or Kenjutsu
What makes them Ninja is that they were used by the Ninja against the samurai, tactics preceding techniques as always
It is not necessary to go into any real depth with these schools early in your career, but they may be referred to in class
this is similar to how a shuto is done different in kempo to kyukushin karate to shotokan and why taijutsu's kiten ken is a unique technique not a variation on karate or kempo's shuto
