They are also said to have brought a pizza cutter and scissors to Bartow Middle School in Florida Tuesday in a bid to overpower and slaughter students younger than them as they came into a bathroom.
After the pair were arrested, police said they discovered days worth of text messages between the two detailing their plans.
Police say the plot was foiled when administrators searched for them after they didn't show up for class. Both girls told police they were "practicing Satanists".
Detectives searched the girls' homes and found a hand-drawn map that included the phrase "go to kill bathroom".
Afterwards, the girls would leave the body parts at the school's entrance before killing themselves, they said.
The girls face charges of conspiracy to commit first-degree murder and possession of a weapon at school, among other charges.
"The plan was to kill at least one student but were hoping to kill anywhere from 15-25 students", the affidavit said.
Mr Hall said that there had been rumours on Monday (Oct 22) that there would be "issues" at the middle school, and a student had informed a teacher that "something bad" was going to happen. "If I had children in school I'd be half scared to death".
It will be up to prosecutors to decide whether the girls will be charged formally as juveniles or adults, Deputy Police Chief Bryan Dorman said in an email.
They told investigators they're practicing Satan worshippers and planned to commit suicide after the attack in hopes it would bring them closer to Satan.
They are then said to have planned to slaughter those children before drinking their blood from a green glass goblet, police say.
In a statement posted to Facebook, Hall said no one was harmed in the incident, and thanked school staff for "thwarting this plot from becoming a tragedy".
The girls remain in police custody, and a school system officials said the school will be filing for expulsion. According to authorities, the girls planned to wait there and corner potential victims, WFTS reports.
The girls were reportedly found with several items the police believe they meant to use to harm others, including a pizza cutter, butcher knife, paring knife, and sharpener.