Police say teachers went looking for them and found them with the knives at Bartow Middle School.
Officers said a student heard the girls planning the attack.
Their aim was then to cut their victims' throats, cut up their bodies, eat the flesh and drink their blood, authorities said.
Bartow Police Department said the pair planned to wait in a school toilet until smaller pupils entered.
Some of the weapons the girls planned to use included butcher knives, paring knives, and a pizza cutter.
Both girls remain in police custody, as parents at the school told U.S. media they kept their children at home on Wednesday after the scare.
Detectives told Time the girls had cooked up the plot over "scary" movies during a weekend playdate.
According to officials, the plot was foiled only after the mother of one of the girls called in when she received a robocall informing her that she skipped school.
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After the girls were taken into custody, authorities reportedly searched their houses and discovered more additional attack plans.
The administrators found them in a bathroom stall and brought them back to their offices where they found the girls in possession of four knives, a pizza cutter and a knife sharpener, police said.
The girls were arrested on Tuesday and charged with conspiracy to commit murder, possession of a weapon on school property, carrying a concealed weapon, and disruption of a school function, Bartow Police Chief Joe Hall said during a news conference.
Two Florida middle school girls reportedly were set to attack their schoolmates in what they allegedly meant to be a mass stabbing spree.
A search of their cell phones revealed messages indicating that they planned to "leave body parts at the entrance" before killing themselves, according to WFTV-TV.
"It is very disturbing to me, not only as an educator but also as a superintendent and as a parent, but I want parents to know we're going to do everything to keep their kids safe", said Superintendent Jacqueline Byrd.
Officials with Polk County Schools said that extra police officers and guidance counselors would be at the school this week.