Ninja Academy

In order for a Ninja Academy student to graduate and become a Genin in the Hidden Mist Village, he or she had to fight, with one of the same friends they trained with, to the death. But all of this had to be changed 10 years ago when one of that year's students, who failed to pass the preliminary exam, went beserk and killed over 100 peers without hesitation. This child was called Momochi Zabuza. After the incident they changed the final exam for the graduating class. In chapter 303 Yamato states that Sai was also subjected to this method of training in preparation to serve under Danzo's Root division.