Ex-49er Bruce Miller's H.S. Coach Says 'Guy I Know Wouldn't Punch Old Man'

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.

O'Brien -- who keeps in touch with Miller and last talked to him a few weeks ago -- tells us, "He's a great kid. Comes from a good family. Cares about people. That's not the kid I know. It doesn't sound like something he'd do."

Coach O'Brien says his primary concern right now isn't Miller's football career (he was cut by the 49ers) ... it's his life.