Police in Burlington, Vermont, subjected a group of high school students to a mock shooting without warning, traumatizing teenagers who have been raised to hide from — and even fend off — school shooters.

In a statement, the Burlington Police Department apologized Thursday for the “presentation” that it staged the day before at the police station.

“The roll-playing scenario only involved three department personnel simulating a robbery scenario and was not directed at any students or faculty,” the department wrote.

Local news outlet Seven Days reported that the simulation involved police personnel bursting into a room and pretending to open fire:

Two students who spoke on the condition of anonymity said the class was facing the front of the room listening to a detective speak when they heard screams from behind them. Two women ran in, the students said, followed by a man wearing a ski mask who was holding a gun. Then [fake] gunshot sounds rang out.

According to local NBC affiliate WPTZ, officials with the school district said teachers were aware that a firearm-related demonstration would take place to show how witness statements can be unreliable, but had not realized that it would happen “without warning.”

Choosing to perform a simulated shooting to teach students a lesson about eyewitness testimony is a baffling choice in itself, considering the very real threat of gun violence that Americans live with. In a time of loosening gun laws and too-frequent mass shootings, active shooter drills are commonplace in schools across the U.S., including in preschool.

Research has shown that such drills can have lasting mental health effects on children, even when there’s ample notice. Gun safety advocates say other school safety measures can be more effective in preventing gun violence — and far less traumatizing to students.