Half of all GCSE English pupils do not recognise that phrases such as "get off of" or "me and my friend" are non-standard English.
Beth Black, a researcher for Cambridge Assessment, the education agency, tested more than 2,000 GCSE English candidates on their ability to recognise colloquial English.
Most pupils were unable to spot when an adjective had been used in place of an adverb. So 41.9 per cent failed to recognise that "come quick" was incorrect. And only 53.8 per cent were able to recognise that "me and my friend" was an unacceptable construction.
Meanwhile, fewer than 60 per cent of pupils correctly identified "off of" or "she was stood" as ungrammatical. And only 58.8 per cent believed it was acceptable to introduce a new noun with the pronoun "this" - for example, "then this man showed us".
Ian McNeilly, of the National Association for the Teaching of English, said: "For a lot of people - not just young people - their daily use of English is in new media, where non-standard grammatical constructions are more acceptable. That's inevitably going to lead to an increased lack of awareness of more standard constructions.
He said language had a history of being used to be inclusive and exclusive, especially in Britain where language and social class are very strongly linked.
Dr Black said: "It is possible that these less well-recognised non- standard English forms will find their way into standard English, especially given the view that teenagers are linguistic innovators who bring about change in standard dialect."
However, 91.2 per cent of pupils surveyed recognised that double negatives were non-standard. Almost as many spotted instances where nouns and verbs disagreed, such as "she walk".
More than 85 per cent also correctly identified as problematic "his mum brung him", "it were quite tall" and "that girl she is tall".
A similar Cambridge Assessment study in 2005 found that GCSE pupils had a better mastery of grammar and punctuation than those a decade ago, but were more likely to lapse into colloquialisms.
The national curriculum states that pupils should understand the differences between standard and non-standard English. But it also calls for teenagers "to be aware that different people make different choices about when standard English is appropriate".
In fact, the majority of pupils surveyed by Dr Black condemned non- standard English, describing the mistakes they spotted as "childish", "chav", "disscorrectly ordered" and "a bit unproper".
"Investigating Non-standard English in GCSE-level Students in England" by Beth Black.