You have a Master's degree and defend an assertion with 'My teacher said!'? Just wow. Anyway, I'll take your request in good faith and expand upon what I said, though I wasn't insulting you and if you really wanted to critically engage with your lecturer's claims, a brief perusal of any etymological dictionary should suffice.
Explaining the origin of a word using acronyms is much too convoluted to survive Occam's Razor. We've got a word for everything, why would some words (and they're nearly always insults and swear words, revealingly) be meticulously formed out of the initials of others? I've had people try to convince me (or ask me to confirm) that "Fuck" stands for "Fornication Under the Consent of the King" or that "Pom" means "Prisoner of His Majesty." These explanations ('backronyms') are appealling on a very simplistic level but it only requires a bit of common sense to see through them. In the case of "Nonce": if a certain kind of prisoner is partitioned from others for his own safety, because it is universally accepted by the rest of the inmates that his crimes make him sub-human, isn't it likely that they've already got a word for the sort of person he is? Or did the cons refer to them as child-molesters, pederasts and paedophiles up until the moment the screws realized their lives were in danger?
From memory, the word "nonce" is much older than the Victorian buildings where it is supposed to have been coined. One of its more traditional meanings is "fool" or "simpleton" which I would suggest is a fairly short step away from its current usage. That's not as tidy an explanation as the backronym, but languages are messy systems that evolve mysteriously over time, rather than secret codes invented by ten-year-olds.
Of course, you could argue that I don't know that your assertion is wrong, any more than your teacher knows it's right (unless he was in the room when the screws invented it, and disseminated the word among his peers -- who, in turn, unthinkingly and somewhat surprisingly adopted Prison Officer jargon as their own slang at his suggestion). But I don't have to look this up to know there will be no reliable source to confirm it. There never is. If you want to live in a world where "Gringo" comes from "Green coat", and to impress the great unread with your 'knowledge', you go right ahead. Sooner or later, though, some former lecturer in Modern Languages will call Believable Until Logically Lessened by Studiously Having It Tested. Or 'Bullshit', for short.
