A school lockdown is designed to keep people from coming in and usually involves keeping students in classrooms for a specified time.
It does not mean locking exit doors from the inside preventing students from getting out. That would be a safety hazard. Think what would happen if there were a fire and hundreds or even thousands of students had no way to exit the building.
That's why exit doors meant for egress come equipt with these.
This Von Duprin model is made right here in Indianapolis.
I've seen some articles showing devices to lock or jam a door that schools could use.
They are designed to keep people out but they never mention that they also keep people in in case of a fire or a shooting.
Some were designed by teachers or school students.
I also wonder how those locks could affect a police response.
They will be slowed down just like a potential killer if each door is hardened and locked from the inside.
Do they just unlock the doors when someone outside claims to be the police?