Cramlington Learning Village in Northumberland has failed its latest Ofsted inspection, the BBC has learned.
The school said the Ofsted report, which is not yet public, was "inaccurate" and that it had "serious concerns" about the inspection process.
The report is expected to be published later. Ofsted has not yet responded to a request for comment.
The school was given the highest rating, "outstanding", in 2013 and in its previous three reports.
The latest inspection gave the overall school the lowest "inadequate" ranking, with the sixth form ranked good, the BBC understands.
In a statement, the school said the report was "disappointing".
Staff and governors believed the report painted a "highly inaccurate" picture of life at the school, it said.
Inspectors had deemed the school had "fallen short" of outstanding status, "however, we have 70% of the same students, the same head teacher and largely the same senior leadership team and governing body as we had at our 2013 inspection", a spokesperson said.
NASUWT teaching union regional organiser Simon Kennedy said: "I think the leadership and the members of staff at Cramlington will be perplexed and baffled how they can go from consistently being an outstanding school to being one where Ofsted grades them as a 'four'."
The school challenged the report's findings but was unable to change the ranking, the BBC understands.
Academy schools are not controlled by the local authority but receive funding direct from government and sometimes through sponsors.
An academy school rated "outstanding" two years ago has been put into special measures by education inspectors.