Mean Moms by Emma Rosenblum

Meet Frost, Morgan, and Belle—a wealthy, gorgeous group of New York City moms, the queen bees of downtown Manhattan. Their children attend Atherton Academy, the top private school in the city, and their social lives revolve around elaborate themed parties. On the first day of school, the arrival of a new mom and mysterious beauty from Miami, Sofia, shakes up their world. When Sofia quickly integrates herself into their clique, inexplicably bad things start to happen to the women. Is someone at school out to get them?
Spanning the course of one eventful school year in New York, Mean Moms is part satire of upper-crust mom-ing and part mystery, interrogating the line between friendship and jealousy, and getting at the question:

What would happen if the woman standing next to you at school pickup was actually a sociopath?