WASHINGTON, March 6, (AP) -- The nation's deadliest school shooting in nearly two years prompted new pleas from safety advocates and education officials for adults and students to keep an eye on youths who seem troubled or threaten violence.

On Monday, police in Santee, California, said a freshman at Santana High School opened fire in a school bathroom, killing two people and wounding 13. Students and an adult acquaintance said they had heard threats from the 15-year-old suspect over the weekend, but thought he was joking and did not report him to authorities.

The shooting was in stark contrast to recent incidents in at least four other communities, where students reported threats and averted violent episodes at school, authorities said. "These are such tragic events, and this one in particular could have been averted if people had come forward in a timely fashion,'' said Gerald Tirozzi, executive director of the National Association of Secondary School Principals.

Since two students at Columbine High School in Littleton, Colorado, killed 12 students, a teacher and themselves in April 1999, authorities have begun taking threats of violence more seriously, asking students to report them to authorities. Among recent incidents in which classmates alerted authorities about threats at school:

In Fort Collins, Colorado, students at a high school in January reported two students' plans to re-enact the 1999 Columbine High School massacre on its second anniversary next month.

In Hoyt, Kansas, an anonymous tip on a hot line last month revealed plans by three heavily armed students to duplicate the Columbine massacre. Authorities said the students had bomb-making materials and an AK-47 assault rifle.

In Elmira, New York, a female student last month received a note threatening violence from a classmate and passed it to a teacher. Police found the student who had written the note in the school cafeteria with a .22-caliber Ruger and a duffel bag containing pipe bombs, a propane bomb and a sawed-off shotgun with several rounds of ammunition.

In Cupertino, California, a De Anza Community College freshman working as a photo lab worker called police in January when she noticed that pictures of a fellow student contained shots of him posing with an arsenal of pipe bombs and firearms. Police said the student had planned a shooting rampage for the following day.

In Monday's shooting, police said the 15-year-old suspect had been picked on and had talked about shooting classmates. In a written statement, Education Secretary Rod Paige said, "I urge every parent and every student to listen closely to children who express concern, anger or fear concerning their school, their teachers and their classmates.'"

Bill Modzeleski, director of the Education Department's Safe and Drug Free Schools Program, said the shooting shows that students can't ignore peers' threats of violence. "When they hear other kids talking about coming to school with a gun ... they cannot be taken lightly," he said. "It's better to be safe and report these things than have others killed or injured."

Modzeleski and others said statistics on violent crime show that schools remain among the safest places in most communities, but school safety consultant Ken Trump said even one shooting, robbery or assault hits families hard. "If it's me or my kid who's a victim, those statistics don't mean a lot," Trump said.

Stephen Sroka, a school safety consultant for the Center for Adolescent Health at the Case Western Reserve University School of Medicine, said many violent students show clear warning signs that they're troubled, including depression, drug use, turbulent sexual relationships and signs they're suicidal. "These people certainly need help," he said. "They don't need help tomorrow, they need help today."