Safety experts have warned of a worrying increase in student threats of gun massacres in American schools.

The arrest of six Alaska pupils suspected of plotting a school shooting rampage brought to four the number of alleged Columbine-style conspiracies foiled by authorities across America in just two weeks in April.

As of last week, there had been 12 reported cases of students either plotting shootings in or making threats against schools since March 1, according to Kenneth Trump, president of National School Safety and Security Services, a consultancy that tracks such incidents.

“It’s become this whole thing, ‘I’m going to pull a Columbine’,” said Edward Ray, Denver public schools chief security officer and president of the National Association of School Safety and Law Enforcement Officers.

The six children, aged 12 to 14, who attended North Pole middle school, just 125 miles below the Arctic circle, were arrested on April 22. The parent of a fellow pupil had alerted police about an alleged plan to cut the school’s power supply, shoot teachers and children, then flee.

The pupils, who face possible murder conspiracy charges, drew up a “hit list” of intended staff and student victims, said North Pole police chief Paul Lindhag. “They felt picked on,” he said.

The planned attack’s timing, around the seventh anniversary of the massacre at Colorado’s Columbine high school on April 20, 1999, was coincidental, Mr Lindhag said.

But Columbine, where two classmates aged 17 and 18 killed 12 students and a teacher before committing suicide, appeared to be the macabre inspiration for an alleged plot interrupted in Kansas on April 19.

The plot was foiled by a child in North Carolina who tipped off police after spotting details of it on MySpace.com, a pupil networking website with a cult following.

Like the Columbine killers, the five Kansas pupils, aged 15 to 18, planned to stalk Riverton high school armed to the teeth and wearing black trench coats, according to a county sheriff’s department official who wished to remain anonymous.

Police found guns and rounds of ammunition at their homes. The students, who have been charged with “incitement to riot” and making a “criminal threat”, planned to sabotage the school’s camera system before opening fire between 12 noon and 1pm on April 20, the official added.

In another apparent Columbine copycat, two 17-year-old students were arrested on April 17 for an alleged plot to shoot staff and students at Missouri’s Platte county high school on April 20.

On April 5, authorities in Atco, New Jersey, interrupted an alleged plot to execute 25 people during lunchtime, also on April 20, at Winslow Township high school. Four pupils face murder conspiracy and terrorism charges.

US schools have come to expect a rise in violent threats in spring - prime testing season, during which many students feel increased academic pressure, and by which time social cliques have formed, increasing student conflict - but the recent spate has left many perplexed.

“We normally see more incidents leading up to the Columbine anniversary and in spring generally, but this is an unusual cluster in a short period of time,” said Mr Trump.

Ted Feinberg, of the National Association of School Psychologists, said:

“It’s common for kids to want to copy events, thinking it will bring them attention and status.”

Pupils feel powerless and see the attacks as a way to gain power and influence, said Bill Bond, former head of Kentucky’s Heath high school, where a 14-year-old shot dead three students in 1997.

Now a school safety expert, Mr Bond is encouraged at least that the latest conspirators have been turned in by other pupils after making ominous comments or threats, breaking an informal code of silence among pupils.

Such warning signs were often missed in previous shootings.

“In 80 per cent of shootings other students knew of the plots,” he said, citing a US Secret Service study. “They are now aware that these things can actually happen and how tragic they can be.”

TEN YEARS OF SHOOTINGS

February 2, 1996 Moses Lake, Washington: Barry Loukatis, 14, shoots dead teacher and two classmates at Frontier junior high school.

October 1, 1997 Pearl, Mississippi: Luke Woodham, 16, shoots dead two students at Pearl high school.

December 1, 1997West Paducah, Kentucky:Michael Carneal, 14, kills three pupils at Heath high school.

March 24, 1998Jonesboro, Arkansas:Mitchell Johnson, 11, and Andrew Golden, 13, open fire on staff and students as they evacuate Westside middle school in response to false fire alarm, killing four students and teacher.

May 21, 1998Springfield, Oregon:Kip Kinkel, 15, kills two students, wounding another 22, at Thurston high school.

April 20, 1999Littleton, Colorado:Eric Harris, 18, (below) and Dylan Klebold, 17, kill one teacher and 12 students at Columbine high school before committing suicide.

March 5, 2001Santee, California: Charles Andrew Williams, 15, kills two students at Santana high school.

April 24, 2003Red Lion, Pennsylvania: James Sheets, 14, kills Red Lion Area junior high school head, then himself.

September 24, 2003Cold Spring, Minnesota:John McLaughlin shoots dead two Rocori high school classmates.

March 21, 2005Red Lake, Minnesota: Jeff Weise, 16, kills teacher, five students, security guard and himself at Red Lake high.