Island police, fire and emergency personnel are planning a large-scale mass casualty drill this Saturday morning at the Edgartown School.

Edgartown police chief Antone Bettencourt said the drill will last about three hours and will likely close part of Edgartown-West Tisbury road and Cooke street.

The drill will simulate the event of an active shooter in the school, Chief Bettencourt said, though police won’t know the details of the incident until they arrive on the scene.

Police from all the Island departments will participate, and each town is sending an ambulance, he said. Emergency medical personnel will simulate assessing injuries on the scene, with volunteers acting as victims.

Chief Bettencourt said a Massachusetts state police helicopter will be involved, as will the Island communication center and Edgartown technology department, which will simulate sending out text alerts. The drill will also involve how to communicate with the media during the incident.

The drill comes after months of planning. Edgartown police, fire, EMS and school personnel have been meeting since September to develop a plan for an incident at a school.

“Now we want to practice what we’ve been putting together,” Chief Bettencourt said. There will be a debriefing after the drill.

Chief Bettencourt said he wanted neighbors to be aware of the simulation so they know that Saturday’s events are just a drill. No elementary students will be involved.

The Martha’s Vineyard Airport does an annual drill simulating a large plane crash, so a drill of this size is not new for the Island, Chief Bettencourt said. But he said this is the first time they’ve done such a drill at a school. “We’re not reinventing the wheel,” he said, but “after incidents at Sandy Hook and other schools you have to look at your school differently.”