By Adam Blenford
When police armed response units pulled up at Scripps Ranch High School, it was bad news for the teenage boy carrying a replica machine gun around campus.
For the student who tipped off the police, though, things were looking up.
Many US high schools have thousands of students
Under an incentive scheme at Scripps Ranch, a large San Diego high school, students can call a special "tip line" - 888-580-TIPS - to report suspicious activity on campus.
Callers whose tips lead to arrests or charges can earn themselves rewards of up to $1,000.
The Scripps Ranch tip was accurate: one 16-year-old student had brought a fearsome-looking gas-powered pellet gun into the school, which he sold to a fellow pupil for $150.
Neither boy seemed intent on criminal damage. Instead, police say, the buyer was probably hoping to find time for a little target practice in the canyons around Scripps Ranch.
But weeks after a deadly shooting spree at a high school in Red Lake, Minnesota, fears of violence in US schools are running high once more.
Under a zero-tolerance approach to weapons in school, carrying anything from a replica Uzi to a simple camping knife through the gates could lead to disciplinary procedures, expulsion and a possible conviction.
Law enforcement officials and school authorities sing the praises of the incentives schemes, which are in operation at about 2,000 US schools.
The schemes are not new, with the first set up in 1983, but they are becoming increasingly popular as students, parents and teachers all focus on security.
Predictably, some are concerned about the effect of injecting a cash incentive into students' everyday lives.
"There are other programmes out there that pay students to read," says Millie DeAnda, chair of school programmes for Crime Stoppers USA, which organises the schemes.
"But in a lot of our schools we have problems where youths are bringing in guns, drugs, knives and if it wasn't for the students none of us would know about it," she told the BBC News website.
"There should always be concern where there is money involved. But we have found that the students know more than the teachers. They are the first to know."
When Jeff Weise killed nine people on a rampage through Red Lake High School, the focus of the police investigation soon turned to the 16-year-old's fellow pupils.
Students at Red Lake did not believe Jeff Weise planned to kill
It later emerged that the son of a local tribal leader discussed aspects of the attack with Weise. Police made an arrest. In mitigation, Louis Jourdain said he never believed Weise was serious.
Kay Herting Wahl, associate professor of educational psychology at the University of Minnesota, told the BBC News website that students often found themselves in trouble for not coming forward with their suspicions.
"Most of the time students don't take things seriously - they just don't believe the things they hear. Plus there is a whole sense of community and that sense of not telling tales on other," said Ms Herting Wahl.
As director of school counselling services in Minnesota, Ms Herting Wahl has direct contact with the Red Lake students.
She maintains that most students would voluntarily report serious issues, including weapons and drug dealing, whether or not a reward was on offer.
But, she says: "Life is more complex than that. They are kids, and their perspective gets pretty warped sometimes."
At Scripps Ranch the authorities love Crime Stoppers.
School Principal Bill Fox sent a letter to parents immediately after the replica gun incident, and sung the scheme's praises in the local press.
"Our number one priority is safety at school," he told the San Diego Union-Tribune. "We encourage any individual who witnesses anything that presents a possible risk to report it. And that's exactly what happened."
Zero tolerance was enforced after the Columbine killings
"I've been preaching it on campus for the whole year," school police officer Dan Palkovic told the BBC News website.
"I actually think it instils a sense of community. Kids do know a lot more than us but a lot of the time they don't want to talk because they could be called a rat.
"Now they can make the call and remain anonymous."
A 10-strong team of armed police were on campus within minutes of the replica gun being spotted in the school on Tuesday, Mr Palkovic said.
Within minutes the two boys were arrested. Their school has now recommended them for expulsion and they face criminal charges for bringing the weapon onto school grounds.
Large high schools in the US routinely have a serving police officer detailed to look after law enforcement issues on campus. Metal detectors and security camera were installed many years ago.
"It's a little like airport security," said Kay Herting Wahl. She recalled how a Minnesota schoolboy vowed to hurt a teacher during a lunchtime fit of rage shortly after the Red Lake shootings.
After thinking it over for a few minutes, his friends, worried what would happen if they didn't speak up, told their teachers.
The boy could barely remember making his threat when he was promptly summoned to the principal's office. He was later expelled, Ms Herting Wahl said.
"It's not something we joke about any more."