6:38 a.m. update: Springettsbury Township Police are asking drivers to avoid Mount Zion Road between The York Galleria and Central York High School Friday morning.

Because of flooding, the staging area for Friday's emergency school drill has been moved from John Rudy County Park to the Galleria, Police Chief Thomas Hyers said. Emergency responders will be using Mount Zion Road and Mundis Mill Road to respond to the drill.

Previous story: National attention will be focused on Central York High School Friday as more than 50 local, state and federal law-enforcement agencies participate in a multi-jurisdictional emergency school drill involving at least one "gunman."

Roughly 1,700 students and 120 school staffers also will be involved at some level, as will local firefighters and ambulance crews, according to Springettsbury Township Police Chief Tom Hyers.

"From what we've heard, it's the largest (drill) ever done in the country," he said.

Nationwide, similar exercises with active-shooter scenarios generally don't include an entire student body, and often involve just a small number of local police officers, according to Hyers.

What makes Friday's drill so interesting is what it does not focus on, the chief said, namely "neutralizing the threat," meaning taking down the shooter or shooters.

New focus: Instead, the drill will focus on the mass coordination between -- and the response of -- multiple local agencies and groups that would be involved in a real emergency, Hyers said.

The drill also will focus on how students and parents are reunified afterward, and how law enforcement should coordinate after the threat is removed, he said.

The scenario involves the school being put in lockdown, followed by those inside

being evacuated, and Central York School Superintendent Michael Snell has worked closely with police to put together the exercise, Hyers said.

The chief said a group of Central High students have started a campaign to have all students wear green and white on Friday -- the colors of Sandy Hook Elementary School in Newtown, Conn., where a gunman killed 20 children and six educators on Dec. 14.

"When I heard that, I'll be honest. I kind of got overwhelmed," Hyers said.

Hundreds involved: The FBI, the state Emergency Management Agency, the Southcentral Task Force and various agencies and school officials will participate, he said. Many others will simply watch.

"Our observer list of police executives and school officials ... is at 161," Hyers said. "That's from all over Pennsylvania, and a few from Maryland and Delaware."

Some Central High students will be more involved than others.

"We're going to take a control group of 200 students who will be involved in the drill," Hyers said. "The other 1,500 will be evacuated as they would be in a real emergency."

The 200 students picked to participate more fully in the drill will have no contact with the "shooter" or the "victims," according to the chief. The victims are members of the community who will be done up in "moulage," or fake injuries.

About 200 law-enforcement officers and volunteers will be involved in the actual drill, plus 35 to 40 firefighters and ambulance crew members, Hyers said.

Coordination: "Counties aren't used to coordinating on this large a scale until a tragedy happens," Hyers said.

The goal of Friday's drill is to allow various agencies and first responders to be prepared to work together, he said.

"Hopefully, as a result of this drill, we will be able to draft key policies," he said. "Tabletop exercises are only so effective.

"We're already much better prepared by just planning for this drill."

The chief said it's his hope such a large-scale drill will be done twice a year somewhere in York County.

Twice a year? "This is not a one-and-done thing," Hyers said, adding he's hoping police can hold a large-scale active-shooter drill at a local mall or factory sometime in the spring.

The public can't simply show up at the school to observe Friday's drill, the chief said.

"We want people to be aware of what's going on, but the school area will not be open to the public," he said.

After 7:50 a.m., access to the high school will be restricted to participants and previously approved observers and media.

-- Reach Liz Evans Scolforo at levans@yorkdispatch.com.