Police Chief Jane Castor said Jared Cano, 17, had threatened to plant a bomb at Freedom High School in north Tampa and detonate the device when classes resume Tuesday.
The suspect was arrested Tuesday night after someone alerted police to the alleged plot.
Castor told a news conference that the alleged bombing plot was planned minute by minute and the intent was to cause more casualties than the 1999 Columbine High School massacre, which resulted in 13 deaths before the two shooters killed themselves.
Castor said police and the school system "were probably able to thwart a potentially catastrophic event, the likes of which the city of Tampa has not seen, and hopefully never will."
At Cano's apartment, police discovered materials to make pipe bombs, shrapnel, plastic tubing and timing and fusing devices, along with a fuel source, Castor said. Police also said they found a journal containing schematic drawings of rooms inside the school and statements about the teenager's intent to kill.
The Tampa high school shares a cafeteria with its neighboring middle school, increasing the potential threat to hundreds of additional students, officials noted.
The plot targeted specific school administrators and any students who were nearby, police said. Authorities did not name the administrators nor would they disclose who tipped them off about the alleged plot.
Cano's mother consented to a police search Tuesday evening, Castor said. In addition to the bomb-making materials, police reported they also found a marijuana-growing operation.
Cano faces felony charges of possessing bomb-making materials and threatening to throw, project, place or discharge a destructive device. Additional felony charges include cultivating marijuana, possession of drug paraphernalia and possession of marijuana, authorities said.
Authorities said Cano had several arrests as a juvenile. Charges have included burglary, carrying a concealed weapon, altering serial numbers on a firearm and drug possession. All had been either dismissed or no action had been taken.
Police Maj. John Newman said Cano had been on a list of juveniles who police checked on from time to time because of their brushes with the law.
"We've been very, very familiar with him," Newman said.
The St. Petersburg Times reported that prosecutors, at a hearing Wednesday, said that when Cano was arrested he repeated his plan to detonate a bomb and cause mass casualties at the school.
The Times reported that Cano tried to speak when he appeared before a judge Wednesday morning but was quickly hushed by a public defender standing beside him.
"The plot wasn't..." Cano said, before the public defender stopped him and told the judge that "he has no comment," according to the Times.
High school principal Chris Farkas said Cano was expelled in April 2010 because of a non-school-related incident and would have been red-flagged if he had tried to come back on campus.
"My first response was shock. I wanted to see if it was a real threat," said Farkas, who is accustomed to unsubstantiated threats from working at a large high school.
"Once I found out and saw the information and saw what was taken from the apartment complex, that was when the reality and the fear set in that this was a real situation."
Police told Farkas that Cano worked alone.
Cano was expelled from Freedom High in April 2010, according to Hillsborough County schools spokeswoman Linda Cobbe.
Last year, Cano attended a charter school and left voluntarily in March, Cobbe said. At that point he was 16 and could have chosen to drop out. He was not registered to attend classes this upcoming school year.
"We're thankful to Tampa police for taking the tip they got seriously and working with us to keep our schools safe," Cobbe said.
Cano's Facebook page includes photos of him holding a machete and drinking from a bottle of malt liquor. He says he attends the "University of Marijuana," where he is studying "how to grow weed."
He lists two favorite quotes: "lessons not learned in blood are soon forgotten" and "dont trust anybody, cuz they all just wait for you to s--- a brick of gold so they can take it." He only listed 25 friends.
The Department of Children and Families said the agency investigated Cano's family in 2009. A spokesman gave no other details except to say DCF found no evidence of abuse or neglect.
Associated Press writers Christine Armario and Kelli Kennedy contributed from Miami.