Ordered to empty their pockets, the 11-year-old pulled out a knife and a knife sharpener, the affidavit said, while the 12-year-old produced three knives and a pizza cutter. The assistant principal walked in after one of the two had been reported missing from class.
Bartow police searched both girls and found several knives, including a butcher's knife, Hall said.
Both girls remain in police custody, as parents at the school told United States media they kept their children at home on Wednesday after the scare.
Investigators say they also found messages on the girls' cell phones about the planned attacks.
They were taken to the principal's office, where the girls revealed four knives, scissors and a pizza cutter under their clothes, police said.
During an interview with officers, the girls told officers they wanted to kill at least 15 people. One such message read, "We will leave body parts at the entrance and then we will kill ourselves", the station reports, citing police.
According to ClickOrlando, the girls face several charges, including conspiracy to commit murder, possession of a weapon on school property and disrupting a school campus. "They were not sad", Hall said.
Two "tween" girls were caught with knives and other sharp utensils at a Florida middle school.
Police say that notices have been sent to the parents of students at Bartow Middle School informing them of the situation.
The youngsters, aged just 11 and 12, brought a butchers knife to school after plotting to overpower their younger schoolmates in a bathroom.
The two suspects were find hiding in a school bathroom moments before they planned to begin the attack, according to WGHP.
"There was essentially no reaction from either of these two girls".
Bartow Chief of Police Joe Hall called the plot "horrific" in a statement posted to social media by Polk County Public Schools. However, I want to make it clear that your children are our most precious citizens here in the City and we do everything we can, every single day to make sure our children are safe.
"The plan worked", she said.