The Liars -- Mona googling with her google-eyes all the while, taking in information about how we hate Meredith, Meredith is the enemy -- immediately form a human shield to cockblock Meredith from doing her job.
Meredith: "Look, bitches. Lotta shit went down, I get it. I had an affair with Aria's dad that led to his blackmail and helped end his marriage, and now we're fucking and he's acting like that shouldn't be weird for anybody. But this is my paycheck you're fucking with, and Civics class isn't about your teen nightmare drama. It's about how the check-and-balance system was established so that no branch could wield absolute power over another branch. Don't start none, won't be none. Got it?"
Liars: "Ugh. Got it."
Mona: "I am going to blow your ass up."
Hanna: "Mona, wait up! Just checking in. And I couldn't leave Aria while Meredith was acting totally reasonable with her after Aria acted like a bitchy child in front of everybody, I mean, that would be an utter betrayal. But I'm here now."
Mona: "It's fine, I get it. I am sure life is real fucking hard for Aria Montgomery."
Nevertheless. Mona opens her locker to reveal... A giant knife stuck in a brain! With creepy lettering that says "TAKES ONE MAD COW TO KNOW ANOTHER!" Which is just, so awesome. I mean really, well done. (And then so as far as public opinion, I guess the story is that Mona went crazy, did some weird things to some people, and got locked up. That works.)
Hanna: "Mona, what is it? Is that a brain? Close the door. Close your locker. Keep moving."
Oh, and she does. Ol' Mona keeps it hoppin', yanking that knife down and walking all the way to the end of the hallway -- brain still on there, yep -- and dumping the brain in a trashcan. Then she stares at all the people in the hallway -- who are amazed by what is occurring, of course, and snapping photos -- and sort of waves the knife around crazily for a while before dropping that in the garbage, whispering a secret thing to Lucas, and then weirding herself out of the situation. By the way, she looks amazing. Schoolgirl chic, with a headband. At least she didn't lose that part of her amazingness.