Secret Clique (Replica 5)
Description pulled from GoodReads: 

Amy Candler never bothered with the popular clique at school. Now she wants to join their ranks. It doesn't make any sense to her friends--especially when a wave of panic is sweeping through the school hallways and friends need to stick together.

Students are disappearing.

Eric is being stalked.

Mr. Devon mysteriously returns, whispering about dark secrets. Everyone at Parkside Middle School is on edge, and even with her extraordinary skills, Amy is about to find out that being popular can be hazardous to her health.
High school students, Identity, Fiction, Middle School, Clones
Marilyn Kaye