Ex-49er fullback Bruce Miller isn't the type of person who'd punch an elderly man ... at least according to Miller's H.S. coach ... who says his former star is "a great kid."
Miller allegedly beat up a 70-year-old man in a San Francisco hotel room this weekend ... before being arrested for aggravated assault, elder abuse, threats and battery.
TMZ Sports spoke to Coach Mike O'Brien -- Smith's coach at Woodstock High School from '04-'05 -- about the heinous allegations against Miller.