Transcript:
Schroeder stands next to Charlie Brown on the field as the latter whistles at Patty as she walks by down the lane.
Charlie Brown grimaces as Patty continues walking by and snorts in disgust.
He watches as Schroeder whistles an entire musical piece at Violet as she walks by.
He grimaces darkly as Violet and Schroeder walk down the lane together, smiling at each other.
Original comic:



Someone else already confirmed it, but here’s a page with some trivia like that:
https://en.wikibooks.org/wiki/Annotations_of_The_Complete_Peanuts/Print_version
It’s kind of hit-or-miss, but does list this one: