Easy A/WMG

Olive is adopted.
After all, we know her brother is adopted, and her father explains that the reason for the adoption is that they were unable to have children. It is possible that they were only able to have one child, but it seems more likely that they weren't able to have any of their own. No one in the film ever says that Olive is the Penderghasts' biological daughter.
 * Taking this one further, Olive must have another, older brother. After all, she tell Rhiannon that she had a date with George, a classmate of her brother's at college. It is established in the film that Rhiannon and Olive had been friends for years, so surely Rhiannon would have known if Olive's only brother were too young for the story about George to be at all credible. So Olive must also have an older brother. It is even more unlikely that the Penderghasts were able to have two children but not a third. So it is probable that Olive and both of her brothers were adopted.
 * Or that they adopted him for a reason other than infertility.
 * Olive's parents tell their son that their "insides didn't cooperate". Which probably means they were infertile.
 * Or it probably means that they got older. Chip is noticeably younger than Olive and people (especially women) are not fertile forever.
 * Or they were just joking. They tend to do that.
 * Note also that Olive says she's only her dad's daughter "by marriage", which could be solely familial banter, or could be a reference to being adopted. Or, so long as we're here, she's her mother's daughter from another coupling.

Marianne is actually Penelope Taynt.
After her beloved show was cancelled and a thorough rewatch, Penelope knew she had nothing else to look forward in life... until her dad found Jesus. Now instead of being a fanatic Amanda Bynes worshipper, she's now a fanatic Christian with absolutely no comprehension of anything else.

Principal Gibbons is actually a future version of Alex DeLarge.
At the end of the book, it's implied that Alex decides to become a relatively normal, law-abiding citizen. And what better way to show that he has completed his Heel Face Turn than by making him an authority figure? And a conservative one, at that.
 * Dude. Someone please write a fanfiction about this. I beg thee.
 * Seconded.

Olive's parents are Amicably Divorced.
Her dad says, "Even when a man and a woman love each other very much, like your mother and I used to." They married, but realized that they weren't suited for one another romantically. However, they remained good friends, and live together for financial reasons, and because they feel it's the best thing for their children.
 * I always thought that line was teasing, but upon rewatching that scene, I found that right after saying that line the parents give each other a weird, not-exactly-loving look.
 * Except they're all cuddly while watching Olive's webcast.

Olive ends up in a position worthy of a driver waiting for her at JFK Airport.
Yeah, we don't have a page for the movie Friends with Benefits yet but in the film Mila Kunis's character takes a sign from a waiting limo driver and writes on the back of it. The side not written on has O. Penderghast printed on it.
 * Emma Stone was Justin Timberlake's character's girlfriend who dumped him at the beginning over their conflicting opinions about John Mayer music.
 * Yes, and she could have just happened to be in New York at the same time as Justin Timberlake's character without knowing it.
 * Emma Stone's character is called Kayla in this film, not to mention obviously being a completely different character. However, there's no reason that she can't just be playing another character, and Olive Penderghast could still exist in the universe.
 * Especially if as suggested above Olive was adopted, meaning Kalya could be Olive's seperated at birth twin.