If you attended a school in Scotland (in the east coast, at least), it’s a sentence that won’t be entirely unfamiliar to you. (For any non-Scot reading this, it means spilling the beans on a misdemeanour to a janitor.)
Scottish playgrounds were, and still are, full of these linguistic oddities. Words are often shortened, warped, or discarded entirely in favour of another, more obscure term.
God help you if you if you were one of these, especially if you were a boy. “Telling on someone” or “grassing them up” by going to a teacher and relaying how wee Jonnie flicked a marker pen at big Neil in class is a cardinal sin: you just don’t do it (unless it’s something a lot more serious, obviously).
The result of a clipe might be the distribution of one of these - a punny, short for punishment exercise. Also referred to as “lines”, so-called because the offender, who would often write these punishment exercises in a classroom after school, would have to write out a self-admonishing sentence (“I should not have called Mrs Meacher a silly wee cow”) again and again, ad naseum.
A janitor is, in itself, a word used widely in Scottish schools, but Scottish children often use the abbreviation “jannie”. As abbreviations go it’s especially informal (mostly because it carries all the gravitas of a freshly squeezed whoopie cushion), and may be viewed by some teachers - and indeed janitors - with a raised eyebrow.
Not to be confused with the more recent phenomenon of vouyeristic car-park sex, dogging was a once widely used term for playing truant (or skipping school, as it was also often called).