The school was visited in February, and now the chief inspector for Estyn - the education and training inspectorate for Wales - has decided to place the school into special measures.
The school now has three months before a further visit to get things sorted, or face further action.
Powys County Council cabinet member for education, Councillor Myfanwy Alexander says the report is not unexpected.
She said: "This is disappointing but not unexpected news.
"The new leadership team at Newtown High School know the steps they need to take to ensure rapid improvement and the authority will be providing all necessary support for the school on their improvement journey."
A report published today said: "Newtown High School is judged to have made insufficient progress in relation to the recommendations following the core inspection in May 2015.
"As a result, Her Majesty’s Chief Inspector of Education and Training in Wales is increasing the level of follow-up activity.
"In accordance with the Education Act 2005, Her Majesty’s Chief Inspector is of the opinion that special measures are required in relation to this school. The school will draw up an action plan to show how it is going to address the recommendations."
The report states no sufficient improvement has been made in key stage four English and mathematics, and there is a lack of progress in other areas.
A further recommendation has demanded that attendance throughout the school needs to improve, and there needs to be a reduction in fixed term exclusions.
Improvements into the quality of teaching and assessments has now been implemented consistently.
A review needs to take place to strengthen the school's arrangements for tackling bullying, however many pupils are confident the school deals with bullying, according to Estyn.
The report also states that a change in leadership has presented challenges, and progress in improving standards has been too slow.
Chair of governors Peter Hough insisted everyone at the school was determined to get the school out of special measures as soon as possible.
He added: "The report clearly shows that insufficient progress has been made since the original inspection in 2015.
"The new headteacher, who started in September 2017, recognises the need for significant change and as Estyn say in the report 'has begun to introduce a series of suitable strategies to address important areas for improvement'.
"There is lots to be done and everyone involved with the school is determined to make the changes required to get the school out of special measures as soon as possible."