A school is taking part in a pilot government scheme to stop slang words being used in the classroom.
Teachers will correct slang in the classroom
If a pupil at Lilian Baylis School in Kennington, south London, uses slang the teacher corrects it on the board and gives the standard English phrase.
They then put a tick next to the slang each time it is repeated to show how often students slip into the habit.
Headmaster Gary Phillips says pupils need to be proficient in both spoken and written English to pass exams.
He does not mind slang used in the playground but not in the classroom.
"We are not trying to ban slang. Children will speak to each other how they wish - that's part of being a child," Mr Phillips said.
Pupil Nicholas Harris uses slang with his friends
"It is a part of their heritage and identity. What we are trying to do is to make them understand that in an exam only standard English will do."
Student Nicholas Harris told BBC London about his experience of the scheme.
He said: "It is hard because the people I hang around with always use slang.
'Tool in their teaching'
"So when I have to speak standard English I really have to focus."
A spokesman from the Department of Education and Skills (DfES) said: "Schools need to respect the integrity of the languages pupils bring with them.
"They must therefore allow the expression of such languages and use these languages as a tool in their teaching.
"However, in order to pass their exams, pupils need to be proficient in both the spoken and written forms of Standard English and this must be taught."