Continuity mistake: When Julie talks to Courtney at lunch, she's wearing a headband and the ends of her hair are flipped out. When she's waiting for the bus, the ends of her hair are curled under. Later on in the scene her ends are flipped out again.
Continuity mistake: At the end when it is revealed that Courtney killed Liz, we see several of the teenagers standing right in front of the stage. As Courtney walks through the crowd several of the teenagers have moved away from the stage and further back in the crowd. They couldn't have moved that fast through the rest of the crowd.
Continuity mistake: When Courtney, Fern and Foxy are in the cafeteria and they're all drinking diet Coke, Fern has her Coke in her hand near her face when Courtney names her Vylett, but when the shot changes to Fern's face, the Coke is on the table. When the shot changes to show all three, the Coke is in Fern's hand again.
Continuity mistake: When the three girls are carrying Liz's body into the house, she is all paled and bruised. When the camera shows her through different scenes the bruises on her body appear, disappear and reappear through scenes.
Answer: They have no reason to follow up the call or never did all they know is that she was at home.