Just remember to make sure it washes off. For some reason, Universities have no sense of humor when it comes to sandblasting.
My brother and his friends suspended an inflatable sex doll from a (phone? I hope) line outside their high school. The Fire Dept. took it down the following morning, but it took them about two hours, and using the full ladder extension, to do it.
The gold-medal standard of “hacks” (“ethical” pranks) is held by the students of M.I.T., in a tradition going back many years. A complete list of their collective hacks is accessible through the link for “by year list”; my fave is the “Great Droid” dressing of the Great Dome from 1999.
Please, PLEASE don’t do this. I work in Conferences at a university, and some students did this to our brand-new dorm 2 years ago, right before finals. They released a ton of crickets, and it wasn’t funny – people had to step on them to get down the hall, which stained the brand new carpet, the kids could hardly live there, let alone concentrate on their finals, and it cost a lot of money to get an exterminator before my department’s guests arrived to stay there. The cost of cleanup, I might add, was passed on to the perpetrators.
So, I guess I recommend that you think through all the possible ramifications before you act.
Now, on to the ideas: my favorite public prank was one my friends played in high school. The principal (not a popular guy) drove a little red Miata. The roof of the cafeteria was pretty low. One day, the principal’s Miata just magically appeared on top of the roof! :eek:
My favorite prank needs a procrastinator who’s writing a big paper. Wait till they’ve been writing frantically for a while (the day before it’s due, of course), and then when they leave the room for a minute, change the font color to white. (Save the file before you do this, just in case.) Hilarity ensues. 
A friend of mine from high school did that to a friend at college. He was in the theater tech department, so I know they did a convincing job…
Another prank pulled off by some friends of mine, this time at Leeds University, involved grass seed. At least three people broke into another guy’s room shortly after he left for Christmas break, spread grass seed on his carpet, and came back periodically to water it. The guy came back to find a stubbly lawn on the carpet.
Gah! Foiled! My mum spotted the sunhat and (her) lip stick in my bag and gave me an odd look. :smack:
This prank is postponed until after my next trip to the op shop.
Pranks I have known:
I have a friend who is virulently arachnophobic. One halloween we took one of those cheap small plastic spiders you buy in bags of 30 and cleaned it up so it looked more real (it was black, so we just removed the excess plastic). Then we hung the spider from some fishing wire at about eye level in front of his door (we were in college) at about 2 AM. Apparently, when he got up the next day…his bloodcurdling scream aroused his roomate from a dead sleep.
He appreciated the sentiment though
My junior year of HS the seniors released a bunch of mice (like 50) in the halls. So for a while you would see a mouse round the corner and then run away. Not particularly well received.
My senior year they dumped a load (like truck load) of sand in the student parking lot and had beach day. Sat outside in lawn chairs with umbrellas…
I have helped saran wrap someone’s cubicle in the office. They wrapped everything individually and then saran wrapped the cube itself.
You could Saran Wrap someone’s car so they can’t get in. Or if you have some friends to hold him in there, out.
Also, one I would LOVE to try is to fill the room with those little styrofoam peanuts. You’d need a lot, though, then you cover the door with Saran Wrap and watch him figure out how to clean it up.