This is BAD GIRL SCENE from the novel The Keeper about mid-book. It's self-contained.
From Write Runner's Bad Girl Blogfest: click
Beth Muenster pranced toward her as Michelle rounded the corner of the Junior High building. Without reasonable cause, the two girls had gone from note-passing friends to hate-mailing enemies a year earlier, in the seventh grade; they'd been best friends since the third. Often together as a result of the monotonous, alphabetic seating ritual teachers were so fond of the first day of school, the two girls had been inseparable from first period to after-school activities. They'd paired for three-legged races, borrowed clothes during weekend sleepovers, shared projects and homework answers, and taken several summer trips with Beth's parents. Her mother had said it was an enigmatic breakdown, to which girls her age were unnaturally prone. Whatever that meant.
Beth stopped a few feet from Michelle, and her blonde ponytail swiped her across the face. She huffed. "What happened to your stepdad? I hear he had a hard night after doing your mom."
"Shut up, Beth. Why don't you go practice your rope climbing." Michelle had started the rumor that Beth found the gym ropes arousing when she climbed, which explained why they were always slippery; this had followed Beth's earlier tasty gossip that Michelle had already slept with a high school guy named Darren.
"Ha. Ha." Beth glared at Michelle.
Michelle turned and walked toward the gym, crossing the bus stop and the alleyway; she had no intention of carrying on with the crazy ex-friend.
Each day this week involved a particular activity for the Physical Education final: yesterday had been running; today it was swimming. Michelle cinched her backpack onto her shoulder and stepped over the curb. Hanna walked beside her, and Michelle held the door for the girl while they walked into the indoor swimming pool area.
Beth was close behind, followed by Jessica and Lindsey. "I heard it was your dad that killed him. The alcoholic's still jealous he isn't getting any of the good stuff from your mom."
Michelle halted. Hanna took a few steps and stopped, turned. "What did you just say?" Michelle said without moving. "Did you just say my dad killed someone?" Michelle swiveled to Beth; their faces were inches apart. "Bitch, I know I didn't just hear you say that, because if you did, I'm about to kick your ass."
"Go ahead and kick it, slut. Here it is. Or do you need Darren to come help you out. He says you give really good head—"
Michelle's backpack slid off her shoulder and down her arm as she swung it. Her hips swiveled as she slammed the pink canvas book bag into Beth's side, leading the angry arc with her thigh, the momentum flowing up her torso and shoulder, ending with her outstretched arms gripping the bag by its two straps like a Medieval bludgeon. It crashed into Beth's left arm just below the shoulder. Jessica and Lindsey jumped back, both faces twisted with surprise and fear.
Beth stumbled toward the swimming pool and grabbed the backpack to steady herself. But Michelle followed the force with her weight, let go of the backpack and shoved Beth—fully dressed, books and all—backward into the pool. Standing too far from the edge to make a clean entrance, Beth's heels caught and she first hit the smooth cement lip around the pool on her rear, then bounced over and fell like a diver from the edge of a boat; Michelle heard a loud crack as Beth's tailbone snapped with the fall. Beth clutched Michelle's backpack with the vain hope it would stop her fall.