King's Manor school in Guildford, Surrey, is to re-open in September as King's College under the management of 3Es Ltd, a non-profit making arm of the City Technology College, Solihull.
However, staff learned this week that, two years after failing an inspection, King's Manor is to be taken off special measures. It is also understood that all teaching at the school was found to be at least satisfactory.
Dick Boland, the south-east regional secretary of the National Union of Teachers, said the news proved the union had been right to criticise the privatisation plan.
He said: "We have said all the way along that, given proper support by Surrey County Council, King's Manor could be turned around. The whole issue of putting it out to tender was motivated by political dogma. It was foolish decision."
However, Stanley Goodchild, managing consultant of 3Es,
said the company had been working at King's Manor since last summer.
It had helped the school devise a controversial new awards scheme, in which King's Manor pupils were told they would not get a place at King's College unless they were well-behaved.
Mr Goodchild said: "I'm delighted the school is off special measures. The staff have worked extremely hard." He added that 3Es had played a part in the school's achievements.
The new school's senior management team had been working at King's Manor since May, and Bob Allan, the acting headteacher for the past year, had been appointed in consultation with 3Es.
Steve Clarke, Surrey's deputy director of education, said that privatisation had been prompted by a shortage of pupils, not the quality of education at the school. The school had not been in special measures when the proposals were drafted in 1998, he added.