Upon detection of an active shooter, either through manual operation or Shotfinder technology, the school is flooded with a safe and breathable smoke agent (CO2, or equivalent).
Visibility in the hallways and spaces (possibly classrooms) is immediately reduced to a few feet/inches. Classroom lockdown procedures could be used in parallel, or a ‘panic button’ could optionally flood classrooms and alert the response team.
Children would be drilled to drop and cover until the situation is resolved, protected by the minimal visibility. Everyone is blind.
A central rapid response team is equipped with thermal/UV imaging to see through the fog.
This would buy time (minutes), and greatly reduce the ability of an attacker, or attackers, to operate.
The solution should be cost effective, at least for large and medium school systems, and especially in buildings that do not lend themselves to efficient patrol and security.