In my limited experience of nail-guns, I thought for safety's sake, it is impossible to fire then in mid air, as it were. The safety-catch reqires that the nose is pushed against the surface to be nailed.
Or is there such a gun as we see used in this movie?
Just so. It could be modified, but not in the few moments allowed for the killer after he picks it up. 'Suspension of disbelief' they call it.
shareNail guns didn't always have trigger catch safety devices (This did come out in 1988 after all and it may be reasonable for it to be a few years old in the school's props department). It also could have had a trigger catch that was already modified or broken. Most likely we are just supposed to suspend disbelief for plot purposes.
I did a quick Google search and all I could find were references saying that originally they didn't have them, but that all nail guns currently sold have trigger catch safety devices. None said when they had them implemented...
I have to say that I saw this with a group of university friends in a theater when it came out and none of us questioned it at the time (though none of us are carpenters or construction workers either).