Scarlet and Ivy (3 Book Series)
From Book 1:

When shy Ivy's troublemaking twin Scarlet vanishes from Rookwood boarding school, Ivy is invited to "take her place." But when Ivy arrives, she discovers the school's true intention; she has to pretend to be Scarlet.

Ivy must think like Scarlet, act like Scarlet, become Scarlet. What on earth happened to the real Scarlet, and why is the school trying to keep it a secret?

Luckily for Ivy, Scarlet isn't about to disappear without a fight. She's left pieces of her journal carefully hidden all over the school for Ivy to find. Ivy's going to figure out what happened to Scarlet. She's got to.

But the staff of Rookwood is always watching, and they'll do anything to keep their secrets buried...

Scarlet and Ivy Series:
The Lost Twin (Book 1)
The Whispers in the Walls (Book 2)
The Dance in the Dark (Book 3)