Sooo...what are Peppermint Patty, Marcie, Franklin, and Schroeder's last names? This discussion was raised on the SnoopyOnelist Mailing List and a few members knew the answers. Though these characters' last names are hardly ever revealed, this shows that Schulz didn't forget to give one to each of them.

Peppermint Patty's full name is Patricia Reichardt. It was revealed when she appeared before the school board (represented by Snoopy, The World Famous Attorney) to protest against the school dress code.

Marcie's last name was given as Johnson in "You're in the Super Bowl, Charlie Brown," as was Franklin's which is Armstrong.

Schroeder's last name is, well, Schroeder. Schulz revealed in one of his anniversary books that he is known by his surname. So, it's Schroeder's first name that we don't know.
Also, way back in the early early days of the strip, Schroeder sent in a request letter to a radio show, and the broadcaster announced "I'm sorry, Mr. Schroeder, but we do not have that record!"