Read General Shehu YarAdua’s book,’A Life of Service’:-

It detailed how they (Shehu & Buhari) were in Class in Secondary School in Katsina, when Captain Jack Gowon (now Yakubu Gowon) came to their school to recruit them when they both were members of the Katsina Provincial Secondary School Cadet Corps in 1961. And they left school the next Monday to join the Army and were sent straight to Sandhurst a few months later, and were commissioned in 1962 (unlike now).