Harris Academy Upper Norwood said it implemented the initiative to allow its students to "express themselves confidently and appropriately".
The words banned at the Croydon school include "innit", "bare", "extra" and the phrases "you woz" and "we woz".
Also, beginning sentences with 'basically' or ending them with 'yeah' will be considered to be too informal.
"In addition to giving students the teaching they need to thrive academically, we want them to develop the soft skills they will need to compete for jobs and university places," the school was quoted as saying in a statement by the BBC.
Students heard using "informal language" will be asked to "reflect" on it.
"It's wrong. You cannot censor a young person's language, they're not talking about words that are offensive, they're talking about some of the words that politicians use," said Terry Victor, editor of the New Partridge Dictionary of Slang and Unconventional English said.
"[The word] 'ain't' was around in the 19th Century, people like Dickens used it...And how many politicians have you heard say "basically" to begin a sentence?" Victor said.
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