A report by the Edexcel exam board said there was "a surprising number of lapses" in standard English. It issued a reminder to teachers that they should discourage pupils from using "street language and text style", adding: "Most answers require formal expression [of language]."
"Many concerns were expressed by examiners about elementary errors, often appearing in the work of apparently able candidates," the report continued.
"At this level it is almost unforgivable for a candidate to use a lower case for the first person pronoun - and yet in occasional answers this mistake was repeated throughout essays." It added that the use of street and text language "appeared with surprisingly regularity in the work of candidates who clearly aspired to at least a C grade".
"Most answers require formal expression but - even when an informal register or style is appropriate - candidates should remain aware of the examination context and, in particular, should not use street language and text style," it said.
There is rising concern about pupils' writing skills, especially among boys. National curriculum test results for 11-year-olds showed boys' writing standards had fallen this year. Only 55 per cent reached the level expected of an 11-year-old by the time they left primary school, the results showed.
Ministers have set up an urgent review of the national literacy hour in primary schools - which is being conducted by a former chief primary schools inspector, Jim Rose. Many educationalists are now arguing that he should also investigate ways of improving writing standards.
The Edexcel report said spelling was "in general inconsistent" and "variety of vocabulary and of sentence structure is often limited". It went on: "Punctuation errors continue to be widespread, with the absence or misuse of the apostrophe a recurrent problem."
However, it added: "Some examiners felt that this year they had encountered an improvement in the overall structure of candidates' writing."
Street words used in GCSEs
* "gonna" for "going to"
* "m8" for "mate"
* "aint" for "are not"
* "wanna" for "want to"
* "u" for "you"
* "shouda" for "should have"
* "i" for "I"