Swansea council's head of lifelong learning, Mike Hughes, said that failure to anticipate a culture clash among teachers had led to a dip in pupil behaviour and results plummeting when a new community-led school opened in the city.
Daniel James Community School was created out of Penlan Boys' School and all-girls comprehensive Mynyddbach in 2001. It was built on the old Mynyddbach site. But Mr Hughes said teachers from the two former schools failed to blend.
Parents started taking their children elsewhere as the school's reputation slipped and a new head has since been appointed.
"The two schools were only about a mile apart but the cultural differences were enormous," said Mr Hughes.
He added that the lessons learnt from the experience led to greater public consultation when the council closed the old St Thomas Primary School in the city to make way for a pound;6 million new one which opened nearby this April.