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A horrifying school gun massacre has reportedly been averted following the arrest of two teenage conspirators.

Police say they were tipped off about the alleged plot to kill staff and pupils by administrators at South Pasadena High School in California.

Officers swooped on the pair after serving search warrants on their homes.

Cops claim the two youngster were conspiring to kill several staffers and as many fellow students as possible at the school.

"Enough evidence was discovered, and based on that evidence the two students were arrested," said South Pasadena Police Department in a statement posted online.

One of the youths, who have not been identified, was detained without incident, while the second student "resisted arrest and officers had to force entry into his residence," police said in the statement.

"He was captured in the home as he tried to run away from the arresting officers," police said.

No further details of the alleged plan to attack the school were given, except that police said the two youths were "plotting to kill three staff members and as many students as possible with firearms."

"This is a prime example of school officials recognising suspicious behavior," police Sergeant Brian Solinsky said in the statement. "It was this information that helped prevent a horrific tragedy."

Police were expected to reveal more details of the arrests and their investigation at a news conference set for Tuesday morning in South Pasadena, an affluent suburban town of some 24,000 residents about eight miles northeast of downtown Los Angeles.

Just over 1,500 students are currently enrolled at the high school, according to its website.

News of the two arrests in Southern California came three weeks after a judge in Minnesota dismissed attempted murder and property damage charges brought against a teenager there who was accused of planning a shooting and bomb attack on his high school. Prosecutors said they planned to appeal that ruling.