In the farewell note leaked to the Shropshire Star, Shrewsbury Academy headteacher Jon Arnold was critical of support he received from the Marches Academy Trust, which the secondary school is part of, and told of his sadness at leaving.
The school, which was taken over by Marches Academy Trust in summer 2019, was placed in special measures after an Ofsted visit in May last year, but inspectors noted that Mr Arnold provided “much-needed stronger leadership”.
In his letter Mr Arnold said: "When I applied to be the headteacher of the school in February 2018, I have never wanted a job so much in my professional career.
"Whilst I was delighted to be able to start half a term early in June 2018, I suddenly walked into a situation that I wasn’t really warned about. More debt than I could possibly imagine, neglected buildings and under-resourced departments, abysmal results, horrendous behaviour and a change of CEO/Trust Board between the time that I accepted the job and the time that I started."
He added: "Some of the progress that we have made as a school has been absolutely phenomenal. This years’ results are much improved, fixed term exclusions are down significantly and permanent exclusions are also dramatically reduced. These things are as a result of your tremendous efforts.
"I have been unhappy for the duration of this academic year and there were many times when I had to dig deep to find the drive to continue under the vast amount of pressure that I was put under."
Mr Arnold will be taking up another headteacher post.
He added: "Whilst I desperately wanted to continue in a school in which I have become so fond of both the students and staff and in which I know has started to turn, I needed to remember how I had felt throughout this academic year and consider whether once our world returned to normal, whether my feelings of unhappiness would also return as my normality.
"In order to try to flush this out and in the hope that I would receive the assurances that I was after as regards the future leadership of the school going forward, I attempted to have some meetings to iron things out. I cannot enter into specific details, but I can state that at no stage was any attempt made to persuade me to stay in post.
"I have a feeling of great sadness presently. It was my intention to lead this school for many years and be part of the team that turned it around."
Marches Academy Trust declined to comment about points raised in Mr Arnold's letter.
David O'Toole, deputy CEO of the trust, will take over as head until a more permanent successor can be found. Including interim headteachers, he will be the fifth person to lead the school in two-and-a-half years.
Sarah Finch, CEO of the trust, said: "I thank Mr Arnold and wish him every success in his new role."
Mr Arnold declined to comment further on his letter when contacted by the Shropshire Star.