Said to put the willies up someone who has broken the rules. The "arrrrrrrr" is followed by the name, form, and any other identifying details of the transgressor, followed by a brief description of the naughtiness and moral judgment from the speaker. Can be within earshot of a teacher. For instance, "arrrrrrrr, Jonathan Lee Blyth, 2C1, of 108 Weaverthorpe Road, you're sniffing glue, that's so naughty". I did get told off for sniffing glue, with two friends. We were sniffing a fucking Pritt Stick. However, between the girl who saw us and said "arrrrrrrr", the form teacher, and the headmaster, that crucial detail was forgotten. The headmaster showed us pictures of a dead boy.

When not in earshot of a teacher, "arrrrrrrrr", or it's regional variation "aaaaaaaahhhhhh" is followed by "I'm getting you duuuuun!" and ostentatious teacher-hunting gestures.
Getting "done" was my mortal fear for most of primary school.

"Am." A Lincolnshire variation on 'arrrrrrrr' and 'ummmmmmmm.'

Errrrmmmmmmmmmmmmm or errrrrrrrrrrrrrrrr being the midlands variation as far as i know. Can also be repeated many times quickly (and prolonging the 'erm' for different lengths of time to emphasise, in a kind of spastic morse code, exactly how naughty the other child has been) so you tend to get something like ermermerrrrmermermermmmmmmmmmmm. In cases of extreme wrongdoing, you might need more than one lungful – in this case, ask a friend to take the baton towards the end of your ermemreermermm, while you pause for air. In this way, a perpetual emememeeemrremrmmer may be maintained without any real effort.

The north eastern version of this was "eeeeeeh".
During playtime at infant school, a girl said this to Crazy Shaun, who promptly punched her in the face and fled the school grounds - we never did find out what he'd been doing to prompt the "eeeeeeh" – it probably wasn’t as serious as punching a girl in the face then going AWOL.

There is also the "Just You Wait Cha Cha Cha", which goes;
Just you wait, cha-cha-cha.
Just you wait, cha-cha-cha.
Just you wait, cha-cha-cha.
.
Repeat to tears.

Eeeesh! is the newcastle variation, not to be confused with errrrr! - a cry of disgust.