The spin-off series you're referring to is called "Fuuma Kojirou: Himeko Hojo no Karmasho." It takes place after the events in the original work by Masami Kurumada and follows Kojiro, his disciples Ko...
The Fuuma ninja clan has been hired by Hakuo Gakuin to safeguard them from Seishikan, which has been causing harm to students who refuse to transfer. This marks the beginning of the Fuumas' journey as...