Edit: DO NOT DO THIS. You'll either not time it right, and have the toilet blow up on you, spraying you with ceramic shrapnel, or you will time it right, and blow an entire wall of toilets off the wall, destroying a fair amount of the school's plumbing.
You definitely shouldn't do this, but you can get gelatin horse pill capsules off Amazon that are over an inch long that could delay it. But don't do it.
Well, I've had to have a plumber clear a sewer incident in a multistorey building. Cause was a chicken ribcage and other chicken bones together with a ton of grease some idiot tried to flush. And to think I thought I'd seen it all...
There's no way this isn't for a public bathroom. Syringes? It's people shooting up and trying to hide the evidence in a public bathroom. This was/is such a big problem at the public library here that the doors were swapped for ones that are like 3 ft tall so that you can't hide what you're doing.