Chronologically speaking, Olive makes her comment about Huckleberry Finn not being applicable to real life afterBrandon runs away from home with a big hulking black guy. Thus it makes no sense for her to say that that would never happen.
It seems very odd that Olive would be sent to the principal's office (by her favorite teacher, no less) for calling a girl a "twat," when the other girl is not reprimanded for calling her a "skank" or a "tramp." Although "twat" is a bad word, it is usually not seen as very serious in America, whereas the other two are much more common and would definitely garner a stronger reaction.
This caused even more confusion outside of the US, particularly to UK viewers, where "twat" simply refers to a stupid and contemptible person.
In the original, R-rated draft of the script, Olive calls the other girl a c***. This was censored and replaced by the most similar four-letter word they could find, so it would be easy for Olive to spell it with her peas later on in the movie.
For the matter, why isn't the other girl sent to the principal's office? She was the one who insulted Olive in the first place.
If it weren't for Olive's confession to Mr. Griffith, he never would have known that his wife had a painful STI, which she could have easily passed on to him. Here's hoping that Olive warned him about this in time.
Brandon was being bullied for being gay, to such an extent that he had to pretend to have sex with Olive, and eventually leave town to stop the bullying. We only ever see him with a bloody nose, but that amount of caution implies that he's been through a hell of a lot worse.