Well, there is difference between the local de-facto norm and the standard language kids are trained to use. Schools in countries with literary tradition will teach the neutral/formal version of the standard variation so that the kids can produce writing that does not look funny (on the other hand, the ridiculously formal version will not get taught either).
They will not tell you, however, that that is far from the only form of language that exists.
A few Russian linguists once said in an interview that no one graduates from school thinking that school maths is everything there is to know about maths. But somehow, kids get the impression that whatever their teacher taught them about Russian (or English) is, in fact, everything the language is.
If you are saying "this is not a glass of milk" (это не стакан молока) it would be correct, but "milk" itself is not something you can count easily (like 2 milk, 4 milk, etc. are incorrect) and therefore using "a" is wrong. Usually you only use "a" if you are talking about solid nouns and not liquids/gases, or things that aren't in plural form.