Staff at the Middlesbrough school, which opened in September, thought the cameras in all classrooms and corridors represented a sinister lack of trust from their managers.
But Nigel McQuoid, principal, said that the teacher's career had potentially been saved by the use of the cameras.
A teacher had been accused of assault by a pupil whose family insisted on seeing a tape recorded at the time of the alleged incident.
The teacher, whom Mr McQuoid supported, agreed to the family seeing the tape. They watched it alongside Mr McQuoid and Hans Ruyssenaars, his deputy. After seeing it was clear that no such assault had taken place, the allegation was immediately withdrawn.
Mr McQuoid added: "This kind of false allegation can drive people to resign, even if they receive the backing of their head, for there is always the fear that others will believe that there is no smoke without fire."
He said this one case had "more than justified" the decision to install the cameras at a cost of pound;100,000.
John Turver, Middlesbrough branch secretary of the National Union of Teachers, said his members had initially been concerned about the cameras, but they were impressed with how they had been managed at The King's.
"I don't think they should be in all schools but the protocol at that school makes it more acceptable," he said.
The principal and the vice principal possess separate passwords and cannot watch the videos on their own.
Mr Ruyssenaars, a former national executive member of the National Association of Schoolmasters Union of Women Teachers, said: "The critical issue is how effective the viewing protocol is so that staff have no worries about the principal using the cameras to spy on lessons. Everyone is delighted about how things have been seen to work in practice."
The NASUWT last week launched a postcard petition calling on the Government to create legislation that ensures accused teachers are not named unless convicted.
The union also presented figures showing that fewer than 3 per cent of allegations of sexual, physical and verbal abuse made against teachers result in convictions.