Dave Battye, a union official, said he was called to the school in South Yorkshire two days into the boy's school career. He told teachers meeting at Eastbourne that the boy was impossible to educate and joined calls for disruptive pupils to be kept out of mainstream lessons.
He spoke as the National Association of Schoolmasters Union of Women Teachers criticised the spread of units to deal with severely disruptive pupils in schools.
Mr Battye, a member of the union's national executive, said: "These are not educational problems and we deal with educational problems."
Delegates criticised plans to set targets for cutting expulsions. The union said school "sin bins" should not be used as an alternative to permanent exclusion.Reuse content