Bad Girls -- Cynthia Voigt

Bad Girls is one of the many books that I just need to re-read occasionally.  Mikey and Margalo are BAD.  They aren't just shy and misunderstood or bored and hyperactive.  They're just bad.  But in a good way.  Think of them as... fifth-grade vigilantes.  They aren't mean; they don't go after people that don't deserve it.  Louis is a bully, Rhoda is just awful and Mrs. Chemsky is a teacher.  So obviously, they all have it coming. 

Mikey is obvious.  She doesn't think: she reacts.  She's angry and strong and she'll punch you if you look at her sideways.  Margalo is much scarier because she's subtle.  She plans.  If you happen to look at Margalo sideways, nothing will happen immediately.  But weeks later, after you've forgotten all about the incident, she'll get you.  She'll get you in a sneaky, underhanded way--so sneaky, actually, that you might not even realize that it was her.

Louis tried to jerk his arm free from her grasp, and - for just an instant - looked furious enough to forget that he was about to punch a teacher.  And that the teacher was Mrs. Chemsky.

Margalo's hand was covering her mouth.  Her eyes were so bright in her face, she looked as if someone was giving her a giant sundae, with three kinds of ice cream and three kinds of sauces, and whipped cream - fresh whipped cream, the real thing - piled so high on top it looked like a ship under sail.

"Oh," Margalo breathed.

Mikey could imagine how embarrassed and ashamed Louis must be feeling.  She loved it.

As Collomia said, it's "a good training manual for all".