Kirkham Grammar School has blamed falling student numbers and an increase in staffing costs for the move.
Unions say staff have been left shocked and concerned for the future’.
The historic school, which dates back to 1549, said fewer than 10 jobs would go, with staff being asked to volunteer for redundancy.
Teachers have also been asked to work an extra hour every week without an increase in pay, the National Union of Teachers (NUT) and the Association of Teachers and Lecturers (ATL) claimed.
It is believed some subjects are to be removed from the curriculum, while infant classes are proposed to double in size.
But acting headteacher Deborah Parkinson said the school was ‘in a very good place’ and ‘very few, if any, compulsory redundancies’ were expected.
Sam Ud-Din, NUT Lancashire secretary said: “From an employee’s point of view, it may feel significant to them. Our members are shocked. Morale is through the floor.”
In a press release, the school initially described its ‘strategic restructure’ as an ‘exciting time indeed.’ But a whistleblower said staff were called to a meeting by chairman of governors Rosemary Cartwright and told of redundancies.
Mrs Cartwright wrote to the ATL two days before and said: “A combination of falling numbers on roll and rising staffing costs have led to our potential redundancy situation.” She also said: “The number of staff affected by this process is very low (less than 10) and we anticipate a voluntary solution.”
Since 2012, the number of students at Kirkham has fallen from 700 to 611, and is predicted to fall further in the junior school next September, it is understood.
Local ATL official Jim Dye said: “We have a number of concerns regarding the proposed timescales, the redundancy selection criteria, and the proposed increase in working hours for staff that we need to have face-to-face meetings with the employer to try and resolve. This is clearly a very difficult time for the school, and we know that staff and parents will be very worried about the situation.”
Mrs Parkinson, who will be replaced by Daniel Berry, current head at St George’s School in Marton, in April, said: “The governors are currently engaged in a process designed to modernise our staffing structure in order to maintain our promise to pupils, parents and staff of delivering excellence in all that we do and to align with curriculum requirements from September and pupil choice.
“Kirkham Grammar is in a very good place and, contrary to what is suggested, anticipates very few, if any, compulsory redundancies.
“We are fully engaged with trade unions to ensure the quality of education and extracurricular commitment enables us to remain the independent school of choice.”