Ion Thomas, head of Welsh at Welsh-medium comprehensive Ysgol Gyfun Gwynllyw, in Torfaen, said that centres need to be set up during out-of-school hours to encourage pupils to use the language in everyday life.
He drew attention to the significant increase in popularity of Welsh-medium education since devolution in 1999.
"Thousands of people believe in Welsh-medium education," he said. "We have created an island of Welshness at Ysgol Gyfun Gwynllyw.
"Now we need confirmation of the existence of the language outside school.
We must promote and encourage pupils to speak Welsh outside the lesson, for example at different activities in the afternoons. You are either faithful to the language and support it, or you are unfaithful."
Mr Thomas was taking part in a debate organised by UCAC, the Welsh-speaking teachers' union, at the National Eisteddfod, in Newport.
Moelwen Gwyndaf, general secretary of UCAC, said: "Pupils see Welsh as a language for school, especially in Anglicised areas. They don't see it as useful."
She recommended twinning schools in predominantly English areas with those in Welsh-speaking communities.
"Children need to visit Welsh areas and see the language used in shops, at home, and in extra-curricular activities. We need to ensure that they take it into the outside community," she said.
But First Minister Rhodri Morgan, who also participated in the debate, said: "The numbers of those learning Welsh through education will have a striking impact on Welsh spoken on the streets and in leisure activities."