During my senior year of high school our school was under construction. The 1950's building was under a major overhaul. During a choir concert, we used the currently-being-remodeled office as a room to change clothes in. I noticed that there was a new fancy electronic box on the wall with a keyring and 3 keys hanging off of it. I opened the box and deduced quickly that it looked like some sort of scheduling system. Maybe a fire alarm or security system. I pulled one of the keys off of it and took down the model number for future use.
Later on I found the key and the number, and did a lookup on the internets and found that the box was the new bell system which played over the schools PA. The old system (consisting of actual bells) was out of commission and I knew they would be activating the new one soon. I grabbed the manual and started teaching myself all of the menus and default passwords to the point that I was confident I could do some reprogramming provided I gained access.
One afternoon, after track practice I wandered through the school and found that indeed the opportunity was there, so I went for it.
Donned my gloves, wandered into the office and found it deserted. My previously stolen key popped it open, and sure enough, they left the default password. I proceeded to get to work on making my own bell schedule. After I was done, I changed the password, closed the door, broke the key off in the lock, and went home.
The next morning I arrived to school a hair on the early side as to make sure I'd get to enjoy it. 7:50am the first bell rang, indicating 5 minutes until class started. I went to my first hour class and took my seat. 7:55am the second bell rang to indicate school has begun. The announcements began around 7:56. At 8:00am, the bells interrupted the announcements. Everyone looked around. "That's strange."
8:05am, the bells went off again, and the giggling began. 8:10am. 8:15am. 8:20am. Every five minutes there was an uproar of giggling going up and down the hallways.
By 11:00am my plan was truly realized, as no one thought it was funny anymore. This was the toughest part of the day for me as I was really enjoying the suffering. I walked by the office during my lunch to see that the principal and technician were still trying to wrangle out the broken key. I knew that they still hadn't gotten to the changed password, and that the reset was difficult to do without the password.
It wasn't until 1:30pm that they finally managed to shut the bells off.