Over the last year and a half, there were about a dozen instances in which students in the district made threatening gestures with their hands, he said.

“To my knowledge, none of the other students have been arrested,” Mr. Smith said.

However, there have been a variety of consequences handed down, the strongest being a three- to five-day suspension, he said, adding that a letter about the recent incident was not sent out to all parents. However, the Westridge principal did speak to the parents of students involved in the matter.

Juvenile incarceration is not uncommon: More than 800,000 minors were arrested in 2017, according to the United States Department of Justice.

Two 6-year-olds were arrested in separate incidents last month at a charter school in Orlando, Fla. One of the children had kicked a staff member, and both faced battery charges that were later dropped. In March, a 17-year-old was arrested in Charlottesville, Va., in connection with a threatening “ethnic cleansing” post online that forced the city’s public schools to close for two days.

In February, an 11-year-old student was arrested in Lakeland, Fla., after he disrupted the classroom and refused to participate in the Pledge of Allegiance. In 2015, a 14-year-old was detained and handcuffed after bringing a homemade clock to a high school in Irving, Tex., that was mistaken for a bomb. He was suspended from school for three days, but no charges were filed. (President Barack Obama later invited him to the White House for an astronomy event.)

According to Elizabeth S. Barnert, an assistant professor of pediatrics at the University of California, Los Angeles, the rate of juvenile incarceration has decreased in recent years, but the United States still imprisons more minors than anywhere else in the world .

“I think the evidence is pretty clear that involvement in the juvenile justice system is harmful, traumatic and should be used as a last resort,” said Dr. Barnert, who studied the relationship between children who were incarcerated and their health outcomes into adulthood.