New!
Try KIKI STRIKE by Kristen Miller. The narrator, Ananka, discovers a hidden room inside of a sinkhole outside her NYC apartment one morning. after that, she slowly becomes pulled into a mystery involving the mysterious Kiki Strike, a girl at her school. (When
Their teacher asks each student to say what they want to be when they grow up, Kiki simply answers, "dangerous.") Ananka joins a crew of former girl scouts who each have a semi-legal special skill (explosives expert, forgery, electronics genius, master of disguise...coolest kids ever) to solve a mystery about a hidden city under new york. (No risque content at all.)
Another great mystery series is SAMMY KEYES by Wendelin Van Draanen. She's a 7th-grader who solves mysteries. She's spunky and cool, the mysteries are good, and there are like a million of them so if she likes them, you're set!
She might also like Maureen Johnson's Shades of London mystery series about a girl who joins up with ghost hunters in London to solve murder mysteries. THE NAME OF THE STAR is the first one. It's set in a boarding school, but from what I remember it's very clean.
GIRL STOLEN by April Henry is a YA thriller about a girl who is blind, who gets kidnapped. It might appeal because of the disappearing-girl element in PLL. Since she's blind, she has to really keep her wits about her to try to figure out a way to escape from a place she can't see. It's VERY suspenseful and really really good! I could not put it down. I've heard good things about her other YA thrillers too.