I went to a catholic highschool and I was very bitter about it. I wasn't even catholic; it was just the only school close enough to our house.
The Father in the school REALLY didn't like me. I never mentioned that I wasn't religious (agnostic) nor did I make fun of it. But I refused to drink the wine and consume the wafer; and I refused to attend the services that were "optional" as in "You can go or you can go kicking and screaming."
He would, every time there was a school service (once a month), sit me in a classroom and make me write an essay on why I wasn't religious. If I didn't he would usually find some way to give me detention in religion class. (I was given detention once for being late to class even though I had a note from the office explaining that I had to update my family's contact information.)
So I decided to get revenge.
All the computers in the school had the usual software to stop students from messing things up. Can't right click, start menu is basically an over glorified shutdown button, etc.
The system however didn't seem to stop you from browsing the entire C:\ drive and any networked drive. No passwords. Nothing. When the computer booted you would see the default Windows XP wallpaper and it would be quickly replaced by one of the school's wallpapers on the network.
I took a picture of Raptor Jesus, saved it over with the same name as the school's wallpaper on the network. Every computer then displayed Raptor Jesus.
We had a school assembly where the principle and vice principle told everyone that they will get to the bottom of this and how "disrespectful" that was. They didn't seem to know the "exploit" (I use this term loosely) and I could still mess with whatever I wanted.
When the computer booted it would copy all the software from some networked drive as well. The school used Firefox as it's web browser. So, this time, I changed the homepage to be the Atheism wikipedia article.
No assembly this time but our teachers all gave us a lecture on how whoever was doing this was going to get in a lot of trouble.
Finally, I discovered that if I connected my laptop to the internet by taking the ethernet cable from a computer and just plugging it in that I had access to the printers now. Normally the software would track who printed what so they could make sure nobody printed off 500 black pages or something.
So I printed exactly 500 copies of Raptor Jesus to every printer in the school.
This seemed to anger them so much that they revoked access to the computers for the rest of the year - save for the courses that required computers.
I felt bad for my fellow students but I don't regret what I did.