Turning all the clocks forward afew hours when they have to get up early for work or anything important. Alot of effort if you have alot of computers ect but well funny.
The old screenshot desktops a goodun.
If your still in school, slightly unplugging someones mouse is funny once or twice.
oooo pressing "alt + shift + print screen" on a windows machine makes it go into high contrast mode. Funny at school where most Teachers dont know about it either and just blame the pupils computer you did it on.
Im in computer mode right now so can only think of computery ways to mess with people

+ I miss school.
[hr]
Edit: Dont really think these are messing with people but...
Wadders said:
A "mate" of mine had a really creepy ability to randomly vomit in public.
I can do that, ive also taught others howto. If you can fake a burp by like breathing air into your stomache, (Not explained very well but its commen, sure you know what I mean) then you just burp it out but keep pushin. Usually your sick within 5/10 seconds of pushin.
I can also fart with my eye. I have no fucking idea how to explain how I do this, and never met anyone else who can, nor have I been able to teach it. Kinda pull the eyelids out and trap air in then push it out it makes a farty noise. Dunno how I found that out and ive worked out how to do high pitch/low pitch aswell
God I feel like a kid right now.