Inspectors deemed that the former Whitecross School, in Lydney, had been failing to provide an acceptable standard of education following their visit in October.
A spokesperson for the school said: "We have now received the final report from Ofsted after the moderation and checking process.
"The Inspectors have placed the school in Special Measures as it has been failing to provide an acceptable standard of education.
"This is a rightly challenging report and the senior leaders and governors of The Dean Academy and the senior staff and directors of The Athelstan Trust agree with its key findings.
"The report recognises the actions that have been taken this year and acknowledges the work that senior leaders and the acting headteacher are doing to improve the school.
"The Inspectors make it clear that the acting headteacher is taking decisive action to raise standards.
"All of the staff at the Dean Academy and in the wider Athelstan Trust are working very hard to ensure that our students receive the education they deserve.
"There are a number of strengths that the report highlights including the care and safeguarding arrangements, the range of extracurricular activities and the teaching and progress in a number of subject areas.
"We will be holding a meeting for parents to discuss the report and hear about the actions that we are taking to raise standards.
"There is a great deal of work to do and we look forward to the support of the whole community."