More than 4.1 million students were in a school lockdown last year

What they did: The Post reviewed 20,000 news stories and scoured data from school districts across the 31 largest cities in the country.

By the numbers: In total, there were 6,200 lockdowns last year.

• A typical day saw 16 campuses placed on lockdown.
• Lockdowns happen in buildings with as few as four students or as many as 5,000.
• Bomb threats accounted for just 15% of lockdowns. 61% of the lockdowns recorded were related to firearms.

The big picture: "The number of students affected eclipsed the populations of Maine, Rhode Island, Delaware and Vermont combined," The Post reports.

Be smart: That number is likely to be much larger than what is being reported.

• Many school districts, including those in Detroit and Chicago, don't track lockdowns and hundreds never make the news — "particularly when they happen at urban schools attended primarily by children of color," The Post reports.

Go deeper: Read the piece from The Post.