After the shooting at Columbine High School in 1999, Congress began providing federal dollars for campus officers, and officials made — and remade — security protocols inside schools, from lockdown training drills to elaborate identification requirements. Nationally, 19 percent of elementary school students, 45 percent of middle schoolers and 67 percent of high school students attend a school with a campus police officer, according to a 2018 report from the Urban Institute.

Still, there is little evidence nationally that the dollars poured into school security measures have decreased gun violence in schools, according to a 2019 study co-written by Jagdish Khubchandani, a professor of public health at New Mexico State University.

“These security measures are not effective,” Dr. Khubchandani said this week. “And they are not catching up to the ease of access with which people are acquiring guns in the pandemic.”