In the strip, not so much the movies and specials, Peppermint Patty, Marcie, Franklin, and I suppose Roy all go to a different school than the rest of the gang. So Charlie Brown may not know what his dog is up to.
For me the silliest arcs are Sally’s “conversations” with the school building (and other strips like it). That requires more suspension of disbelief for me than any other storylines in the strip. But to each his own, of course.
Snoopy is quoting Scottish politician/patriot Andrew Fletcher in the last panel. Basically Fletcher’s thought was there would always be a Scotland as long as there is Scottish music, regardless of what England did. So the joke is Snoopy uses a quote, like most of his legal quotes, that makes no sense in the given situation.
My personal view is Schulz felt it was more respectful for Franklin to be a mostly generic character than for him to attempt to write about an experience he wasn’t familiar with, given his own background (a white man born in Minnesota in the 1920s).
Jimmy would probably like the caterpillar . . . for a snack!