Pupils at a West Midlands school have been banned from talking or writing in their Black Country dialect.
A letter sent to parents of pupils at Colley Lane Primary School in Halesowen said it was introducing a "zero tolerance" approach.
Staff have been criticised for thwarting attempts to keep the Black Country heritage alive.
Head teacher John White said the letter was only meant in the context of classroom teaching.
He added this was not "an attack on local culture".
He said: "The Black Country is a fantastic region with wonderful history and we're absolutely clear that this is not about damaging that in any way. It's about getting the best for our children.
"If children are using certain phrases, that can be confusing for them because when they come to spell that word, they are not saying it in standard English and that can hold them back."
The measure is accompanied by a guide explaining to parents the reasons for the ban.
The list of 10 banned words include prohibitions on saying 'you cor' rather than 'you can't'.
Mr White said literacy was the school's "biggest challenge" with 40% of the intake on free school meals - a measure of how many disadvantaged pupils there are in a school.
The head teacher said he was seeing an increasing number of pupils coming through nursery with little or no proper English, and put slipping standards of language down in part to "a reduction of conversations around the dinner table" at home.
He added teachers had spoken to colleagues at schools in Bradford and south London, where similar methods have been successfully employed.
Voice coach Sharyn Collins praised Mr White for the move: "I think the head master is brilliant and really brave."
But Stephen Pitts from Cradley Heath-based Black Country Tee Shirts, which produces clothing with Black Country phrases on them, said the rule was a setback to attempts to promote the dialect.
"It definitely does have a detrimental effect because it puts that stigmatism towards it," he said.
"You can't get rid of the Black Country dialect. It's bostin'. When people talk about heritage it is inherently wrapped up in the language."
He said "attitude, desire and work ethic" determined how successful people are - not accent.