As promised, I'm penning a small entry about what happened in Lana's head to make her turn against Matt.
The first thing that influenced her was the source. Marlene wasn't just a girl from school. This was her best friend. She trusted her implicitly and when Marlene told her that it was a fact, it broke her heart, but she wasn't about to call her a liar.
The second thing that was tied up in this decision was that she'd had a classmate commit suicide the previous year when she found out she was pregnant after being mistreated. When reminded of that event, Lana has a history of turning vicious and violent.
Third, Matt was acting guilty. The truth was that he was filled with self-loathing over what he'd done to Miranda, but everyone else was dismissive of whether he should feel guilty over that.
Put the three of those together. A boyfriend who's disproportionally guilt-ridden in the opinions of those around him, an alternate reason he might feel guilty from a "reliable" source and Lana's visceral hatred of people who victimize girls and is it really surprising that she went off the deep end?
The other girls have slightly different reasons, but at several points in Book 2, they point out that Lana had been so certain of his guilt that she convinced them. Beck's her sister and her reaction set off a completely different one in her. She was Matt's best friend. She trusted that when he told her that she and Lana were his first that he was telling the truth. He'd never mentioned hooking up with Tricia to her before, so as his friend, she'd expect him to be straight with her.
Tabby went through something similar. She had a miscarriage, rather than an abortion, but the thought that he could victimize another girl like that turned her stomach.
Lilly is Lilly. Her character develops significantly in Book 2 as the reader gets a better look into her motivations, but for now, it's not the first time she's taken a secret and sat on it.
Charlotte is the one that's going to take most people by surprise. She was aware that he was supposed to have been involved with a girl. Her role in his life for the past six weeks comes into more focus as the book wears on, but she felt like he was becoming more and more like his father when the news reached her ears. None of the kids told her, but being a teacher in the school, she heard it third or fourth hand after things had gone horribly wrong. It did a fair bit of damage overall.
Collie's motivation for leaving and keeping the code of silence is perhaps the most heartbreaking of all. I won't spoil it, but she acted from a different mindset than any of the other girls and when it comes out, the reason becomes a blow to Matt's life all over again.
I hope this settles some of the doubts that a lot of people have when they read the first few chapters. There's a LOT of revelations left to see the light of day and each one changes the focus of that breakup a little more. I promise, more answers are coming.