abc
Are you ABC? Watch out! What ABC stands for depends on your response.

Say yes, and you've confessed to being an African Bum Cleaner. Say no, and you've just denied that you are A Brilliant Child

This doesn't exactly work if you're going to be a dick about grammar. And it is, to be fair, only really funny if they say "yes", because that's the only way you get to say "oh Jesus you're an African Bum Cleaner, this is most irregular".
written by Be* Arn*ld, approved by Log

Also to add to your "abc" definition, it was used to denote the exchange of an "already been chewed" piece of gum from one kid to another.
written by Mi*e *, approved by Log

a b c
1 2 3
Now I get
To punch thee
Prelude to punching someone in the chest, or the face if you like. Can be prevented by getting a punch in first - like most things, really.
written by ro* h*rris, approved by Log

The victimisation of Carlisle's Ben Hodgson reached a peak when the "Anti Ben Campaigners" formed. Not just your average group of Ben-hating teenagers, we had a string of successful singles, the best-remembered being "Ben is a Bummer".
Our group was forced to retire after one of our cassettes was passed about on a school trip, and a teacher listened to it.
Apparently, a group of boys dedicating every spare hour to writing songs about Ben bumming things was not only bullying, but it said more about us than it did about Ben.
written by S *, approved by Log

In the 1980s, Northampton Borough Council workers drove around in yellow vans with 'NBC' on the side. We would therefore taunt the school fleabags by chanting "your dad works for Northampton Bum Cleaners".
Twenty years later, I now make a living cleaning bums in a Northampton old peoples home. The irony is sickening.
written by Ma*k P, approved by Matt

See PT. Yes: "You're a Pregnant Teen?" No: "You're not Potty Trained?" Got 'em.
written by Ju*tin*Jaf*e, disapproved by Log

This was also a form of kiss chase where a. might denote a kiss, b a cuddle and c a kiss and a cuddle. The definitions were decided by the girls and then the boys chased them. It took me a while to realise that I wasn't just unlucky, but that they made it up as they went along and didn't want me to touch them. Funnily enough, this game was never used as a way of teaching algebra.
written by excluded pupil, disapproved by Log

unfortunately i went to a middleschool called "arch bishop cramner" or ABC for short.. so you can probably the average 11 year old response to finding out that they were due to play the ABC tabletennis team
written by excluded pupil, disapproved by Log

African Bum Cleaner in schools in the north of Lancashire. The band with the same name caused some mirth.
written by Ya* Spi*, disapproved by Log

Someone with the misfortune to own a car with ABC in the numberplate would also be granted the title of 'African Bum Cleaner'
written by To* C*ambe*s, disapproved by Log

Another letter trick is "Are you TP?"

Yes:Pregnant Teen
No:Potty Trained
written by an*nymo*s *ser, disapproved by Susan