I think the meaning of the word less has evolved - the '9 items or less' being the best example. It had never occured to me that that sounded wrong but 'less apples' does sound wrong. As such I wouldnt actually view it as a mistake, just as a difference in language.
For example many of my friends say 'i will be with you momentarily'. To an English person (which I am, as are my friends) this means 'i will be with you for a moment, then I will leave again', to an American this means 'I will be with in a moment' and this is the way they are using it. They are not using it incorrectly, just in a different way to the way we are used to hearing.
I also agree with the poster who commented that it is used in this way in maths. Could a number be seen as a singular? A single entity? It makes sense to me to say 'is this less than 9?' I think it is this use that confuses the issue and makes the word evolve.
I think you grammar is most influenced by the way you speak and the generation difference between you and your children will always bring up difference. Yes standard English is important but the fact less is used in this way so regularly suggests to me it has become standardised.
Oh also, if you tell the teacher that a numeracy worksheet has a (debatable) grammatical error he/she will pretend to care and then never give it a second thought.