Read this strip at gocomics.com.
Patty doesn't mean to drive Charlie Brown to hysterics, but it is kind of frightening to think about. The space-filling overtelling of one of the characters here is acceptable, as it improves the joke.
It struck me just now that gender relations in Peanuts are already surprisingly equalized. This is far from stereotypical girl behavior. Patty gets used more as being a foil for Charlie Brown than for being female. Maybe girls are considered to be more impish, and that explains why Patty is happy to point out C.B.'s mistake in the first panel; it's hard to imagine Shermy being happy there.
Look at that expression of fear in panel 3. It's really a kindness that Charlie Brown doesn't realize that he'll actually be in school for 46 more years.
