Police and teachers, their nerves raw after a series of shootings in which pupils have killed 10 students and two teachers since December, are taking drastic action against threats.
They are cancelling public events, locking the doors of their classrooms, holding emergency drills and arresting students who threaten violence or even joke about it.
The number of violent threats has swelled, partly because of copycat behaviour by students coming to the end of their school year and partly because of increased public attention, officials said.
A 15-year-old boy from Springfield, Oregon, is accused of killing two classmates in a shooting spree at his high school late last month. He is also accused of later shooting his parents at their home.
Just two days earlier, a student in Tennessee was charged with fatally shooting a classmate who was dating his ex-girlfriend. There also have been fatal shootings at schools in Pennsylvania, Mississippi, Kentucky, California and Jonesboro, Arkansas, where two boys aged 11 and 13 shot dead four girls and a teacher on March 24.
"I want the same mentality that they have around airlines now," said Dave Metzen, superintendent of schools in a suburb of St Paul, Minnesota, where a sixth-grade student was suspended for saying he thought the Oregon school shooting spree was "cool" and he was going to make the same thing happen at his school. "If you joke around about a bomb on that plane, it's over - you don't get on the plane. I think the message has to be sent to kids that you don't joke around about this stuff."
Other schools have toughened security over the past two weeks in the wake of the shootings. In one West Virginia county, the education authority brought in police to patrol its nine high schools and monitor students. One school called off its graduation ceremony after a bomb threat - no devices were found.
During a surprise safety drill, teachers at one California high school were ordered to lock their doors while administrators monitored the school's grounds and stayed in touch by two-way radio.
Teachers in an Ohio high school began locking their classrooms after a student allegedly made a list of more than 20 students and teachers he wanted dead. The student was arrested.
A Kansas town called off the last two days of classes because of a rumour that a student planned to take a gun to his school. Concerns for safety prompted another Kansas school to switch several end-of-year events from the evening to daylight hours.
A dance at a Massachusetts high school also was cancelled when a student threatened to bring a gun and "make it look like Oregon". The principal said teachers were too scared to attend. The 16-year-old student was placed under observation. Where once they could have expected nothing worse than a suspension, students who make threats now face arrest.
A 16-year-old Ohio boy was jailed for six months for drawing up a hit list and threatening to harm more than 20 pupils at his school. Three other Ohio teenagers faced charges for alleged involvement in planting pipe bombs near an elementary school.
Also in Ohio, a 15-year-old student faced criminal charges for threatening to bring a machine gun to school and "shoot everybody". He later said he had been joking. "These kinds of things will be taken seriously by police," Chuck Wilson, a police spokesman, said.
A 21-year-old was charged with carrying a .22-calibre pistol strapped to his ankle when he attended a high-school graduation ceremony in Missouri. He claimed he carried the gun for self-defence.
In Delaware, two boys aged 13 and 14 were arrested for allegedly pointing what looked like a handgun at students and teachers' aides in the playground as a joke, threatening to "blow them away". And in Pennsylvania a boy was arrested after levelling a toy gun at a teacher.
"From now on we'll be paying attention to what's being said in the playground and halls," said Max Riley, superintendent in a Connecticut town where an eighth-grade boy was arrested because he had allegedly threatened to take a bomb to graduation. "Jokes about violence, about threats, they're not going to be taken lightly any more."