Donald G. Walters, 18, of Winfield, Mo., 50 miles northwest of St. Louis, was charged with first-degree conspiracy to commit assault in the alleged massacre plot.
Police said Walters had crafted a plot to shoot as many students and school employees as possible, pointing to a note he wrote on his computer that said, "I hate everything and everyone; I wanted everyone to die," the St. Louis Post-Dispatch reported.
Walters' plan was allegedly stopped after two Winfield High School students who Walters had twice approached to create death lists tipped off authorities, police told the newspaper.
The students first thought Walters was kidding, police said.
Walters allegedly admitted to the scheme, saying his plan was to run through the school and shoot as many people as he could, police said.
He allegedly was first going to attack the school's resource officer, get her gun and then use it to kill the others, police said.
No weapons were found in Walters' home, police said. Walters has no known previous arrests.