Orkney Islands Council, which previously made no comment on what were seen as internal staff matters, has now confirmed it has received the letter and will pursue the issues it raised.
The letter from 40 of the 63 staff at Stromness Academy, reporting "outrageous bullying behaviour", was sent to the chief executive of Orkney Islands Council, Alistair Buchan.
It is understood the allegations relate to both staff and pupils, and the school's senior managers and teachers on temporary contracts were not asked to sign.
The letter claims staff had "reluctantly come to the decision that action is required" against a "situation we have been labouring under for the past few years, as we go about our everyday business in the school."
Claiming an "atmosphere of fear and oppression", the letter alleges staff have been denied "dignity at work" and that their opinions and suggestions are ignored.
Mr Buchan has now responded, saying: "Orkney Islands Council takes the wellbeing of all its staff and service users, including pupils, very seriously.
"In situations such as this, it's important that the Council takes time to investigate matters in a full and fair manner, supporting all the parties involved and giving them an opportunity to express their own views as part of the process."