Public schools in Flint, Michigan, on Monday began enforcing a ban on backpacks, even those made of clear plastic material, that was enacted over concerns about firearms, weapons and threats.

The prohibition, approved unanimously April 25 by the Flint Community Schools Board of Education, came after district officials locked down and then closed Southwestern Classical Academy for a day in April amid unspecified threats.

Last week, a high school student in Knoxville, Tennessee, was arrested after a gun he allegedly had in a backpack accidentally went off in a classroom, resulting in minor injuries for a teacher, authorities said.

And in January, a 6-year-old is alleged to have taken a handgun to his Newport News, Virginia, elementary school in a backpack before he used it to seriously injure a teacher.

“Across the country, we have seen an increase in threatening behavior and contraband, including weapons, being brought into schools at all levels,” the district said in a frequently-asked-questions brief for parents.

“Backpacks make it easier for students to hide weapons, which can be disassembled and harder to identify or hidden in pockets, inside books or under other items,” it said.

The school board decided on a fairly straightforward backpack ban that prohibits those made of transparent plastic. The ban applies through the end of the school year.

In a letter to parents and other constituents last week, Superintendent Kevelin Jones argued full prohibition was the best way forward.

“We have thought long and hard about this decision, knowing that it will impact how scholars and families prepare for their days and operations in the classroom,” he said.

In February, the district surveyed students, parents and staff members about limiting backpacks to clear ones. “This alternative does not completely fix this issue,” the district concluded, according to the FAQ.

“Weapons can still easily be hidden in clear backpacks,” it said.

The new policy states that small purses for personal items, clear plastic bags for gym clothes and lunchboxes will still be allowed “within reason.” They’ll also be subject to searches, the 11-campus, K-12 district said.

Alvin Hamlin, the father of an Eisenhower Elementary School sixth grader, approved of leaving the shoulder-strap bags at home.

“It’s basically for the safety of the kids,” he told NBC affiliate WEYI of northern Michigan. “You have kids bringing guns and stuff to school now.”

Fellow Eisenhower Elementary parent Tyanna Martin said she also approves of the new policy but with some reservations.

“You can hide anything anywhere,” she told WEYI. “I would be still concerned, because anything can happen.”

A key to stopping violence on campus, Martin said, is having students who know better. “I just hope the kids learn to make better decisions than what they are doing,” she said.