The entire joke here rests in Violet saying "let
him play by himself" instead of "let
Schroeder."
There are some nice touches in this one. In addition to the musical staffs that Schulz spent so much effort on, and he put a G-clef instead of an S in his name in the first panel. But best of all we have Snoopy in this strip for no story-related reason other than just being cute and funny there on his end of the couch. His reaction in panel 6 is best here, he's just rendered so winningly in that pose, exactly halfway between a dog-like and a human reaction. It's great.