Ninjas were among the undercover servers in japan. And despite what many people will tell. History about ninjas is scarece. up until 15th century.
The ninjas unlike the samurais could've come from any socio-economy class. As a result Ninjas created few schools of teaching. And learned how to use what ever came in to their hands. Be it a Trowel, a scythe, and like the samurai their choice of weapon was the Katana
Schools of teaching
Like the name suggests, The ninjas had many cultures. from pesants, to royal dynasties. As a result a few school of teaching to ninjas has came.
Current existing schools: