Short answer - read Elmore Leonard. Read him deeply and take notes on how he constructed his dialogue. His craft is usually so excellent that you won't really mind if his work isn't your thing (it isn't mine, but I still love him). The man was a master of dialogue, and probably the very best example of vernacular and regionalism used correctly.
I second this rec. I think Lajos Egri may have mentioned transition through dialogue, too. I've also transcripted exchanges from movies and real life and compared them to see what's consistently omitted from the fictional conversations, and occasionally do a writing exercise in which I jot down an exchange and then build the story around the dialogue.
Bing Z said:
"As you know, Blair," Mary Sue says, blowing so-called white widow pot smoke out of her mouth and wearing bright red lip gross on her super ugly lips that makes her look more like a cheap hoe than a straight-A student headed to Harvard. Her pot smoke does indeed smell potent but that is poo-like instead of weed-like. This girl has no shame. "We used to be BFFs but not anymore. Not since you stole my guy. F-star-star-star you. Go to da hell, da hell, hell hell hell hell hell. And don't even think about stealing William Joshua Hamilton The Third from me. He's actually gay so you can totally forget about it.
Add to that pile of pet peeves the Unintentional Infodump Non-Conversation, in which the author has more than one character rapidly disgorge information the readers need in the same scene, resulting in a "conversation" in which none of the lines have anything to do with each other.
Intentional non-responses can be brilliant ways of showing the characters aren't listening to each other or are misunderstanding each other, and I love those, but I mean things like this, to continue your example:
"As you know, Blair," Mary Sue says, [...]"We used to be BFFs but not anymore. Not since you stole my guy. F-star-star-star you. Go to da hell, da hell, hell hell hell hell hell. And don't even think about stealing William Joshua Hamilton The Third from me. He's actually gay so you can totally forget about it. "
"Of course, Mary Sue, ever since the academy broke with the uniform tradition," Blair responded, "Some students have embraced it as freedom to display personal style, which has caused rifts between the students who liked the old way better, and I worry this will all come to a head at the prom."
"I got a flat tire last night, so my car is in the shop and will probably still be there at an inconvenient time, causing us much consternation later in the plot," Mary Sue retorted.
"Fortunately, the school day has been extended to give us a longer holiday," Blair replied wistfully.
...okay, that's really exaggerated, but even subtle versions of this bug me. It sounds like a pair of malfunctioning Stepford Wives.