On Tuesday three students were shot and wounded outside an elementary school in Washington D.C. A day earlier, three teenagers were shot as they left their high school in Philadelphia. Last week, there were three shootings at high school graduations in Michigan, Louisiana and Tennessee.

    Gunfire at U.S. schools is at a record high, with a gun being discharged on a school property almost every day this year, according to David Riedman, a researcher who tracks such incidents.

    There have been 137 shooting incidents at schools so far this year - almost one a day - and 249 last year, according to Riedman, lead researcher at the K-12 School Shooting Database at the Naval Postgraduate School's Center for Homeland Defense and Security. It tracks every incident in which a gun is brandished or fired or a bullet hits school property.

    Before Tuesday's mass shooting at Robb Elementary School in Uvalde, Texas, other shooting incidents on school grounds around the country had killed 27 people, including seven students, with 77 wounded, Riedman said.

    Those shootings were the 2012 Sandy Hook Elementary attack, in which a gunman killed 26 children and school staff; the 2018 shooting at Marjory Stoneman Douglas High School in Parkland, Florida, which claimed the lives of 17 people; and Tuesday's massacre in Texas.

    The threat of deadly violence hangs over American school children in a way it does not in most other countries. Starting from a young age, students regularly practice active shooter drills, learning to hide in classrooms with the lights off and doors barricaded.

    As a daily precaution, some schools not only lock the front doors but also classroom doors, hoping to block potential gunmen who make it into the building. In Texas, there is even a school marshal program, where trained teachers can carry handguns.  read more 

    The increase in shootings has spawned a schools safety industry. Many schools have installed bullet-proof windows and doors, special locks, metal detectors. Some have hired armed guards.

    But that often is not enough as guns continue to make their way onto campuses with deadly results, Densley said.