The General was in the same chapter as the Doctor in school, despite joining later than the Doctor.
Relationship with The Doctor
Nathan (The Doctor), Tylerhathan (The Hunter), Koschei (The Master) and him were all best friends in school, when the Doctor left Gallifrey, he stayed, but later, when The Hunter was infected with parasites, he saved him along with the second doctor.
It is unknown how the General regenerated from his first body to his second, it is only referenced in The Tale Of The Angry Man, that he has regenerated in the first place. He was forced to regenerate into his third body for the Time War, so he would have a fresh body for the upcoming battle, he regerated for a thrird time after he was hit with a bolt of lightning by The Master. (Time Crack)