This glitch can be done on Halloween (or via mods)
Anyway, I tried this out on Wii so I don't know about PC/Xbox/PS2.
I found this out by chance.
You need to be on the Halloween mission.
Complete that as normal, and do The Big Prank.
If you complete that, DO NOT change out of your Halloween costume and just run around Bullworth Academy until 1:30AM then hit a girl/little kid and LET the prefects bust you.
If done correctly, you'll spawn outside Dr Crabblesnitch's office in your Bullworth Uniform.
Just walk around and you'll notice EVERYONE will still be in their Halloween Uniforms.
Just get busted every night at 1:30am -2:00am when everyone is wearing Halloween outfits.
To end this glitch, just go to sleep as you normally would.
I think people will spawn randomly while the glitch is active, I remember seeing Donald, Algie and Fatty near the gym along with Bryce, Gord and Tad, and Lefty, Peanut and Hal.
Also whilst this glitch is active you CANNOT attempt ANY classes or missions.
Also the weather may be very foggy.
This glitch IS NOT harmful. I don't recommend saving while this glitch is active though.