The customs related to telling on someone are many and varied. As if you didn't know that.
Rather than dwelling on the many ways that tattle-tales are disparaged, let's have a look at some of the customs assosciated with actually telling.
At my school, when someone got in trouble, one would say "um mum mum mum." Sometimes this was done in a sing-song voice (each note getting higher before dropping off at the end), and sometimes it was a hastily spoken "um um um." I've no idea where this came from, whether it was just a local thing, or what. It's a tough one to look up!
Another popular custom in my town was to announce that you were going to tell on someone. When asked why, you would reply
Because you put ants in my pants
and made me do the boogie dance
If you were really pushing it, you might add:
all the way to France
This certainly wasn't unique to my town, and surely pre-dates my time, though I can't really trace it very well. The oldest book I could find it in was from 1986, which is roughly when I would have heard it. I have a theory that most people hear the initial couplet, and the third line just sort of writes itself, roughly the same way, all over the place, but maybe I'm just going out on a limb.
Where and when did YOU hear this one?
And did people say "um mum mum" when someone was getting in trouble in your town?