Somewhat uncharacteristically, Pig-Pen gets angry at the way people refer to him here. It's a bit difficult, through all the grime, to read his expressions of ire. I'm not quite sure I get this strip though -- I sense there's something about it, maybe some context from the time, that I'm missing. I'm not actually sure the girls are judging his appearance, although if they're not then why would Schulz use Pig-Pen here?
I think Schulz spelled it "SKWEEK" in the third panel just to mix things up a bit. We get another funny drawing of Snoopy here, who is already the most plastic of the Peanuts characters.
My favorite thing about this strip is the slight irregularity in Snoopy's jaw in the third panel, indicating Snoopy chewing. Lucy's mouth seems to be missing in the first panel.