something is going VERY wrong if a child gets to school leaving age and doesn't realise that you can't spend more than you earn.
Really? You can't? Do you live in a world where there are no credit cards, no mortgages, no loans, no credit agreements? You very clearly can spend more than you earn, and often it's widely regarded a good idea. I mean, you might believe mortgages are a bad idea because they allow you to spend a hundred grand on a house when you haven't got a hundred grand, and that the only way you should buy a house is for cash.
Making those judgements is potentially complex. And school teachers, based upon coming from a teaching family and hearing horror stories about my parents' colleagues, are no better at that than anyone else.
Teaching "don't spend more than you earn" as a budget maxim is like saying "never mind about contraception, just don't have sex, it'll all be OK". Reality is a lot more complex. If all parents could teach contraception, there would have been no unwanted pregnancies over the past thirty years. If all parents could teach about budgets, no-one would have gone bankrupt over the last thirty years, no-one would have bought PPI and no-one would have been mis-sold a pension (something teachers, as it happens, were particularly prone to).
I'm not sure teachers are qualified to teach this sort of stuff from their life experience (source: know teachers) but I'm bloody sure a lot of parents aren't.