The exception is the group of parents who are linguists. Modern linguists hold it as a basic tenet of their scientific study of language that there is no such thing as correct English, and, therefore, the statement 'Standard English is grammatically correct English' is at best meaningless and at worst prejudice.
Linguists would say that there is an endless variety of English: the type people use in Liverpool, the type we use for talking to babies, the type for science, for writing and so on. Linguists have held this view since the beginning of this century.
How, then, has it happened that almost the whole population believes, in many cases passionately, the flat-earth idea that there is one correct English? One reason is that for most people, other ways of speaking seem alien. Another is that we have believed for centuries that there is a correct English. A third is that language is as personal as the shape of someone's nose, and people do not take kindly to criticism of either. The main reason, however, is that linguists, while believing that English is variable, still write about grammar as if it were monolithic.
Open any grammar book and you will find scarcely any mention of the extraordinary variety in English grammar. Even well-known variations such as 'I done nothing' are excluded. Lay people will find nothing wrong with this because they 'know' that the only grammar is standard grammar. But linguists know that 'I done nothing' is part of English grammar and ought to figure in the grammar books.
That it does not means there is a hole in the knowledge of English grammar as presented to us by grammarians. In recent years teachers of English have been trying to get us all out of this grammatical hole. But the new syllabus will dump us right back in. The only solution is for the language experts to start filling this vacuum with knowledge about English in all its variety.
Then, teachers will have the means to stand up to prejudice and develop their pupils' home English to the point where it can be easily understood by speakers from other regions.
Pupils will learn to use English well and recognise when others use it well if teachers are allowed to show them how to make the best of their own English.
The writer teaches English as a second language.