Slumber Party Massacre

Trish Devereaux (Michelle Michaels), an 18-year-old high school senior, decides to throw a slumber party while her parents are away for the weekend and their neighbor Mr. Contant (Rigg Kennedy) is given the job of checking in on the girls during the night. She awakes to the sound of her radio and gets dressed shortly before going to school. Meanwhile, escaped mass murderer Russ Thorn (Michael Villella) with a preference for power drills, kills a telephone repair woman (Jean Vargas) and steals her van. Trish meets up with her friends Kim (Debra Deliso), Jackie (Andree Honore) and Diane (Gina Hunter) and the girls on her basketball team. A new girl named Valerie Bates (Robin Stille) is invited by Trish, but refuses after hearing Diane talking cruelly about her. Russ Thorn watches the girls leave school from the van and a girl named Linda (Brinke Stevens) goes back inside the school to retrieve a book for a test, only to be locked inside and attacked by Thorn who damages her left arm. She eventually hides in the shower room, but the killer finds out where she is due to her blood loss and shortly kills her before escaping to the van.
That evening, the party and the bloody decimation of the girls begins, as they smoke marijuana and talk about boys. Valerie lives next door and is babysitting her younger sister Courtney (Jennifer Meyers) while their recently-divorced mother is away for the weekend with a new boyfriend. Diane's boyfriend John (Jim Boyce) and two boys from the school named Jeff (David Millbern) and Neil (Joe Johnson) arrive and spy on the girls undressing. Thorn attacks and kills Mr. Contant with his power drill; meanwhile Courtney is begging Valerie to go to the party, but Valerie protests. Diane begins to make out with John in the car and after she gets out to ask Trish permission to go with him, she comes back to find him decapitated. Diane tries to flee, but is murdered also.
While the girls are on the phone with their coach, Mrs. Jana (Pamela Roylance), the pizza guy is shown with his eyes drilled out. Coach Jana hears the girls screaming and calls Valerie to check on them and decides to drive over to the house to check on them herself. The girls try calling the police, but Thorn cuts the phone line before they are connected. The teens arm themselves with knives and Jeff and Neil try to run for help, but are gorily killed by Thorn. Russ gains entry to the house, murders Jackie and chases Kim and Trish upstairs. Courtney and Valerie go over to the house, but find the house dark, unaware of the horror that has happened. Trish and Kim have barricaded themselves in Trish's bedroom. They hear Valerie and they ignore her, thinking she may be the killer's friend. Thorn, unexpectedly enters the bedroom through a window and disarms Kim, stabbing her with her own knife and causing Trish to flee and hide.
Courtney and Valerie enter Trish's house and find Kim dead. Thorn attacks them and Valerie escapes to the basement while Courtney hides under the couch. Coach Jana arrives and beats Thorn with a fireplace poker, but he quickly murders her. Trish manages to stab Thorn with a butcher knife, but it barely slows him down. Valerie chases Thorn with a machete out the back door, and severs both his drill bit and his left hand in the process before he falls into the swimming pool. As the girls embrace, Thorn arises and attacks the girls before getting impaled onto the machete by Valerie, killing him. During the aftermath, Valerie and Trish break down into tears upon killing Thorn and Courtney looks on in a state of shock as police sirens are heard in the distance.

Question: Why did Linda go back inside the school?
Answer: Retrieve a book