The first incidence of shit writing in my experience was someone writing shit, in shit, on the toilet wall.
"What's that?" someone could ask.
"Shit," their friend might have replied.
"I can read, thanks, I meant what is... oh."
A week later the words "GUNS 'N' ROSES" appeared on the same wall. Not spelled out in Guns and Roses, though. Still shit.

An actual excuse, as written on a late slip, for arriving half an hour late to school. I was on that bus, so I should know - it was we who had pushed him over the edge by ringing the bell every two and a quarter seconds and singing songs about vaginas.

At assembly, we used to sing the song 'He's got the whole world in his hands'. This would infuriate the piano player, who would slam the piano lid shut and scream "it's HAND, damn it, he's got the whole world in his HAND". Which obviously invited the question "well what's he doing with the other one, miss?".
I think she just hated plurals, because she went even more mental when she did the same thing to Would you cross over the other side, if someone called for aid.

Take a look over here if you want to get hold of this and other scary-sounding safety information films.

Why not get the family round to watch the vaguely-titled "I think I need to use an isotope?" Or baffle at the rather gay "Don't tell the lads" and "Mind your back!"

"Smell my cheese", the bully would invite. Cheese famously smelling delicious, you would eagerly bend over to the waiting fist, anxious to see if there is a tiny cube of fragrant cheese concealed within. As you get closer, you become suspicious. There's no cheese here... and then, the bully would punch you in the nose. A pleasing variant of this is when the bully adds "Smell my cheese, would you?" and walks off huffily, as though you've offended him mightily. You are the victim of another imaginary foodstuff. See also "You just drank my wee".

One particularly bored lunchbreak a gang war broke out. One of the school wags had stolen a box of chip forks (the pointless little wooden chip eating implements), and after a football-match-based-argument, the said pupil formed a gang called the Chip forks (I was Chip fork number 9). His rival, not to be out done, formed a gang called the Hoopies (I don't know why they were called this). Hoopies would catch Chip forks and draw large H's on their foreheads with the indelible markers. Eventually, over a number of days, the whole school became divided into Chipforks and Hoopies, and registration after lunch was brightened with the sight of a sea of Blue H's on foreheads (long before Red Dwarf existed). Great days...

A particularly obnoxious fart that moves throughout the room, causing as much panic as a bubbling puddle of liquid AIDS.

Serene was a very stupid girl, several years younger then me. She was of Asian descent, had a disproportionately sized head, and was very, very stupid. She would be teased by people asking "Serene, are you stupid?" to which she'd reply "Yes". She'd get asked that question up to twenty times per break time. She never once cried.

I actually saw a bogwashing happen once, to a young lad named Ben Tovey. Have you changed your name? Are you running from something? Are you trying to make the idea of bogwashing seem so whimsical and made-up that no one will ever accuse you of having been bogwashed? Huh, Ben? Well?

This deserves a reply. Come on, Alan. Are you really Ben, as Tom alleges? Did you get bogwashed, dammit? And if anyone else wants to ease the pain of those bogwashing memories by sharing them with the rest of the class, let us know forthwith. - Matt.

I witnessed a severe case of peanutting in which the victim, rather alarmingly began to turn purple, choke and cough up blood.