CENTER VALLEY, Pa. -

From Newtown to the Navy Yard, it's a grim scene we know all too well -- a mass shooting.  But what if it happened here?  Emergency crews trained for the unthinkable Sunday morning at DeSales University in Lehigh Co.

"We're actually simulating a campus emergency," said Mary Ellen Miller, a nursing professor at DeSales.

"The best way to practice this stuff is a situation like this," added student Philip Szalczinger.

It starts out like most nightmares do.

"The dispatch was a pulled fire alarm," said student A.J. Cook.

An ambush no one is prepared for.

"Not everyone knew the situation ahead of time," said Szalczinger.

Cook added:  "They found out that it was an active shooter in progress."

Around 35 nursing and physican assistant students trained, with another 30 or so playing the victims.

"It was pretty intense," said student Kyle Kromer.  "We had a student that was gagging up blood."

The training was emotional, realistic, and hands-on.

"They're actually assessing the victims," said Miller.  "This type of experience is something that you can't get in a traditional classroom setting."

The "victims" are colored-coded for severity.

"You have to treat the patients who need help and evacuate as soon as possible," said Kromer.

Cook added:  "Once people started coming out, it got pretty hectic."

There were mistakes, of course.

"Number Three, we shouldn't have had the shooter as a patient," DeSales EMS' medical director told students after the drill.

But then again, how can you really prepare for the unthinkable?