But if you can hear it coming, and your walls are fairly tough, tip the desk over to use a weight leaning against the door, and the big guy (there is one in every room, and most of us are cranky) sits/leans against it to keep it from sliding. Makes it very hard to open the door.

And now two of my friends have community service hours with house keeping at my ala matter. They tested it last night it seems- a 60 pound desk + 150 pound skinny guy = a door that isn't going to open unless you torch it open or make an explosive breach. I'm not going to point out that while they might have blocked the door, it won't stop a bullet, but they both read about Horatius or the 300 when they were kids.

Of course, I might have wedged the desk in the corner so that if you apply leverage it jams against the wall, and pennied the door, but....