http://www.bbc.com/news/world-europe-35093799The teacher said he had been attacked while alone in a classroom in Aubervilliers, a suburb of Paris.
But prosecutors said he had wounded himself with a box cutter and was now being questioned as to why he lied.
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The 45-year-old teacher - who has not been named - has been treated in hospital for superficial wounds to his side and neck.
According to his account, a man had attacked him with a box cutter at about 07:10 (06:10 GMT) on Monday and had shouted: "This is for Daesh [Islamic State]. It's a warning".