When the kids think Charlie Brown is a genius and are fighting over him to help them with their homework, Shermy pushes Franklin out of the way and says, "I saw him first!" In the very first comic strip way back in 1950, Shermy sits on the sidewalk with Patty and says, "Well! Here comes ol' Charlie Brown!" So within any form of Peanuts media, Shermy literally saw him first. (If this was done intentionally; otherwise just an awesome coincidence.)
And what could either be another coincidence or an obscure Mythology Gag, Patty being attracted to Pig-Pen might be inspired from Pig-Pen's debut appearance in the comic strip on July 13, 1954, in which Patty was the very first of the regular cast (of that time) to meet and got to know Pig-Pen.
When Peppermint Patty gets a perfect score on the standardized test by marking the bubbles in a smiley face pattern, it seems ridiculous that just randomly doodling on a test would receive a perfect score. But, considering that the whole thing is a satire on the inaccuracies of standardized testing and the fact that it is a poor measure of intelligence, it makes more sense. Linus even lampshades the futility of standardized testing before the test.
In a nice Mythology Gag, Peppermint Patty refers to Snoopy as "that funny-looking kid with the big nose". Later, however, she calls up Charlie Brown to tell him what his "crazy dog" is doing. Continuity error? Maybe not. It would be fully in-character for Patty to be aware that Snoopy is a dog but be unable to see through his Joe Cool "disguise".
In a way, the bird in Woodstock's gang who always messes up, while he's meant to be comic relief, is a foil for Charlie Brown. While the former has much less screen time than Charlie Brown, both are underdog characters who want to prove themselves and gain the acceptance of their peers. Where they differ is their outcomes. Charlie Brown works hard to succeed, and while things go wrong as usual, they seem to be strokes of bad luck (the model plane destroying the book report), and he is eventually accepted by everyone, thanks to The Little Red-Haired Girl's reassurance. The bird, however, just wants to show his older teammates that he can help, resulting in his screw-ups being accidents on his part (moving the rope that landed on the other pole, leading to the remaining planes crashing). In this case, the other birds respond by showing disdain toward him, specifically by glaring at him, whacking him with their hats, and laughing at him. Even at the end at Snoopy's victory party, only four of Woodstock's fellow birds (not counting Woodstock himself) are present celebrating, with the outcast bird being nowhere in sight.
On careful viewing, the unlucky bird is actually among the group at the final party. He can be recognized as he is the only one of the birds who is always wearing his cap backwards.
In every Imagine Spot of Snoopy as the Flying Ace, the bottom portion of his doghouse is always out of frame or obscured from view when flying. That's exactly how the doghouse always appears in every comic strip and TV special when Snoopy's airplane is "in the air".
Charlie Brown's main problem is lack of self-esteem. He had made a good impression on the Little Red-Haired Girl since at least the Winter Dance before showing his moves (she's seen looking at him while he keeps the door open for Marcie, and stares for a while), but he doesn't know until the very end because he's just too shy to try and just talk to her.
The Kite-Eating Tree helping out Charlie Brown is a lovely, Heartwarming moment, but it's possible the Kite-Eating tree was repaying an old debt. In one strip, Charlie Brown actually fed the Kite Eating Tree his kite, feeling sorry for it. Since Charlie Brown had decided to throw the Kite-Eating Tree a bone, it may have well be reciprocating.
Linus and Lucy aren't twins, yet are shown to share a classroom with Charlie Brown. This is technically possible, though unlikely. Here are three ways how
Linus, given his frequent flashes of astounding intelligence, could have been moved up a grade. Incidentally, this happened to Charles Schulz in Real Life.note During early elementary school, he was "promoted" (Sarcasm-Quotes his, for the record) two grades. Having to spend his entire youth as the smallest and weakest kid ended up crippling Schulz psychologically for the rest of his life and gave rise to Charlie Brown's personality.
Or perhaps Lucy was born right after a school's cutoff for separating gradesnote (in America, depending on the state or school district, this is usually sometime in September), and then Linus was born in the summer of the next year before that year's cutoff. That puts them in the same grade without Linus skipping one.
Or they're in a split-grade class their town does seem small enough for that to be a possibility. In the strips, Miss Othmar was Linus's teacher, so it's not a stretch that she might have taught both him and Charlie Brown's class together.
War and Peace, according to Shmoop, goes on and on several times about sexual desires. Charlie Brown was reading this, out loud, in front of Sally and no doubt plenty of adoring fans, and the mwah-mwah parent(s) (that presumably exist) did nothing to stop it. note Also, either Marcie read it too and said it was the greatest story ever written, or somebody else told her, and maybe the rest of her class, that it was the greatest story ever written.Charlie Brown is eight years old, and the other kids are no older than nine.