A free school has failed to enter any Year 11 pupils into end-of-year examinations, in what inspectors called an "unreasonable and unorthodox" step.
The Route 39 Academy said its students were "neither academically ready nor sufficiently mature or resilient" to take GCSEs, according to an Ofsted inspection.
The school in Bideford, Devon, said it "strongly refutes" the judgment of the inspection which deemed the school inadequate and placed it into special measures.
Route 39 Academy has 138 pupils on its roll and had a two-day analysis on June 21 and 22 this year.
The inspectors' report concluded the school was failing on all four criteria it measures.
It said: "The decision to hold back an entire cohort is an unreasonable and unorthodox one.
"Leaders and governors state that the pupils in Year 11 are neither academically ready nor sufficiently mature or resilient to have taken the examinations.
"They state that, had they taken the examinations at the usual time, these pupils would have significantly under-performed and attained at low levels.
"This is an admission that the school's curriculum and its delivery have not met the needs of pupils by ensuring that they make good progress and are ready for the next stage of their education, employment or training."
Inspectors added the failure to enter pupils into exams was a breach of "the statutory requirements and the school's own funding agreement".
The governing body of the academy said in a letter to parents: "We strongly refute the judgment and the manner in which the inspection was handled.
"There was a disproportionate focus on our decision to enter 13 Year 11 students for GCSEs in 2018, and a lack of adherence to Ofsted's own Inspection Handbook.
"As a result, we have challenged Ofsted and have issued a complaint."
The school said the pupils who did not take their GCSEs this summer had experienced an "unsettled Year 7 at other schools and wanted a new start".
"Some parents felt that their children - who came to Route 39 Academy with low attainment - would benefit from an extra year academically and emotionally," the letter added.