Ms. Othmar allowed the book report to be submitted late without penalty.
It appears that there is no sign that Charlie Brown got into trouble for the lost book report, beyond his own depression.
So, Linus and the Red Headed Girl told Ms. Othmar about the accident and the teacher realized that she made a mistake assigning a book report to a student who was absent with permission. So, the Red Headed Girl, touched that Charlie Brown did that much work on her behalf alone, was allowed to write one on her own and it was credited to Charlie Brown as well for a decent grade.
Peppermint Patty really does care for Charlie Brown.
And not just in the sense that she has a crush.
Like the Little Red-Haired girl, Patty would've seen Charlie Brown's positive qualities, and fell for them in a similar fashion. Between her personality being so different from Charlie's (she's much more assertive and confident, for one), and not being the brightest bulb in the hardware store, it's not unbelievable that it never occurred to her to point these qualities out to Charlie Brown himself, simply because the idea that he might not already know would be too foreign. Furthermore, she'd been one of the witnesses to Charlie and the Red-Haired Girl's final conversation,
and after seeing that she managed to put a smile on Ol' Chuck's
face in such a short amount of time, something Patty herself could rarely manage, decided to leave well enough alone,
at least for the time being.
Fifi is real and is the Little Red Haired Girl's dog.
Not only that, but she's also her closest friend just like Charlie Brown and Snoopy. The reason Fifi is never seen with her in public is because she is shy around others.