This issue was addressed in a very early draft of the screenplay...
They all exit, leaving George sitting at his desk. George is thinking about
something. He opens a desk drawer and pulls out a scrapbook... not an ordinary
scrapbook... a HYDRAULIC SCRAPBOOK. It says so right on the cover, along with the
name of the manufacturer: “E. Brown Enterprises.” George plugs it in, and pneumatic
cylinders begin automatically turning the pages. At a particular page, it stops.
George stares at the newspaper clipping on the page.
INSERT – CLIPPING
A story with the headline, “Police Quell Near Riot At School Dance,” along with a
photo of the dance that shows Marty on stage!
GEORGE stares at the photo, then shakes his head.
Nah. Couldn’t be.
INSERT – PHOTO
But it is....
ROLL END TITLES OVER NEWS PHOTO
And, of course, who is to say they DIDN'T notice and comment on it, but we didn't see it? I mean, what would they have said anyway?
Lorraine: (Staring at a young Marty) Wow, Marty! You look just like a highschool friend of ours. (To George)
Honey, c'mere! Look at Marty. Doesn't he look like that friend of
George: You mean Calvin? Oh yeah, he does! Shame we don't have any
photos of him. Honestly, if he hadn't had disappeared all those years
ago, I'd have sworn you'd been cheating on me!
Lorraine: Oh, honey! As if I'd cheat on you...
George: I know, dear. (*kiss*) Is Biff done with that car yet?
Remember that Calvin was only around for a few days in 1955, and Marty was born 13 years later. What would YOUR parents say if you resembled a childhood friend they hadn't seen in 30 years?