It's called a slide stop. Close the slide by pulling it all the way to the rear and releasing. Won't close if there's an empty magazine in there, nor should it.

Using the slide stop as a slide release is a fine motor skill that will betray you in crisis.

But it sounds like you guys are trying to administratively close with an empty magazine in the pistol anyway, hardly a critical task. Close the slide. Insert the empty magazine.

-- Chuck