I really did this in college.

I was a tour guide, and also working in the computer center during the summer, and so was another friend of mine. A third friend was also a tour guide and we knew his schedule.

Our raised floor had almost 18 inches of space beneath it--enough to get oneself completely under the floor with the tiles shut above you.

So, a few minutes before we know that our friend would be taking people through the computer center (we knew the route and how long it would take), I got under the floor and waited. I could hear the tour come through, and once they did, I started yelling, "Hey! Let me out! I'm stuck under here!" The tour was confused, and the other friend in on the joke did a good job of looking stunned and confused, and saying, "Mike, where are you?" and I'd say "Over here!" until he took the floor puller, lifted the tile, and I popped up like a jack in the box, dusted myself off (the under floor area is horribly dusty), pretty much directly in front of the tour (about fifteen people), and thanked Tom for letting me out. The tour guide looked at us with a look of shocked disbelief with a bit of, "Yup, I'm not surprised."

I don't know if any of those prospective students actually came.