What barracade ?.... they were plastic windows
Nope. It was in immediate reaction to the shot.
The barricade of all the chairs and furniture stacked up on the inside of the locked doors.
The intact window were almost certainly bullet resistant.
Everyone was surprised at the shot, because there was no warning.
In a one shot situation like that, everyone is trying to figure out what happened.
At first, you do not even know where the shot came from, for sure.
It takes seconds to process what is happening.