Showing The Tangled Web by Kathryn Reiss

Julie really likes the new girl in her class, Carla Warner. Still, there's something odd about her. The things Carla says don't quite add up, and she seems to avoid answering certain questions. At first Julie is sure there's a sensible explanation, but as Carla's stories become more outlandish, Julia can't escape a disturbing fact: either her new friend is lying--or she's in real danger.