Three judgments by inspectors about the effect of national funding policy on a Warwickshire school were removed from the report, which was withdrawn by OFSTED and re-issued.
But one, describing the serious financial position of the Coleshill school, had already been sent out to parents in a summary by the inspection agency.
Warwickshire County Council is now demanding an explanation from Chris Woodhead, the chief inspector, who sanctioned the removal of the comments. The original report has not been withdrawn, pending his answers. Eric Wood, county education officer, said: "The issue raises grave concerns about the editing of OFSTED reports."
Jane Rochelle-Towle, head of the school, was told by OFSTED that the original report and summary had not been approved by the registered inspector. But the authority believed the findings represented the views of the team, led by an OFSTED accredited inspector.
Coleshill was inspected in October. The contract was given by OFSTED to the Open Book agency.
The original report said the school gave good value for money, despite per capita spending being below the national average.
Mrs Rochelle-Towle was sent a revised copy, which had several judgments omitted, including comments that the school operated in difficult financial circumstances which "derive from external political factors rather than from internal management actions".
A spokesman for OFSTED said the first report had been altered by someone in the agency without the registered inspector's approval. "We intend to look into how this situation arose and take whatever steps are necessary to ensure that it can't happen again," he said.