Best pranks I've done are totally not replicable, they were made up on the spot.
My all time favourite has to be this:
In high school the age-old excuse for not having home/coursework was 'I just need to print it'. You'd then get to leave the lesson, find a computer room and spend a frantic 15 minutes finishing it, then printing it. However, if there wasn't enough time to complete it you'd simply go to the computer room, waste a good ten minutes and return saying it wouldn't print because of <insert excuse>. I had a mate who genuinely had done his coursework and really did only need to print it off. He left the class room and headed to the PC suite. While he was gone I saved my own mobile number back into my phone under his name and sent myself a text saying 'Haha I haven't even done it, I'll just tell her it wouldn't print.' I then went to the teacher and said 'Look miss, I know I'm not meant to have my phone out but I thought I should show you this:' <text from Joe Bloggs saying he was going to lie to her>. Thinking this was as far as it'd go, and that he'd waltz back in with homework in hand, I sat back down waiting to be MythBusted. In a gazillion-to-one chance the printer was genuinely f*cked and he really couldn't print his homework! He said to the teacher 'Sorry miss, it wouldn't print…' She hit the f**king roof! She was like 'How dare you lie to me?! You've already had two extensions yadda yadda! I saw the text!!'' … 'What text?!' … 'Don't act all innocent! See me at the end of class!' I ended up coming clean, but only at the very last minute. Teacher saw the funny side; mate did not