The Kemnal Academy Trust (TKAT), which runs the school in Tilgate, said it was undertaking a “restructure”, which would take effect in September.
The National Education Union (NEU), which represents the majority of staff at Thomas Bennett, said TKAT planned to make Â£1m of cuts which, if they went ahead, would see 20 per cent of the leadership team would go, along with 15 per cent of teachers and around 30 per cent of support staff.
There were also fears that the school's sixth-form would close from 2020.
TKAT, however said the figures were “not accurate”, adding “the scale of the proposed restructure will be determined once the consultation period is over”.
A spokesman added: “After exploring all other avenues, we have made the decision to undertake a restructure at the Thomas Bennett Community College.
“Our priority, as always, is both to ensure high standards of education are maintained and that we are doing our utmost to support all members of staff.
“We are currently in a consultation period, meaning we are working with individual members of staff to come to the best possible resolution.”
The spokesman said the consultation included looking for roles for staff within TKAT as well as working with agencies and speaking to other local schools who may be looking for staff.
He added: “The restructure, which will take effect in September 2018, enables the school to remain open and the teaching of our pupils to continue.
“Once the consultation period is over and next steps have been determined, we will be able to share further detail on the nature of the changes.”
Glenn Kelly, NEU regional officer, said: “TKAT must withdraw this reckless proposal and get round the table with the union and community representatives to discuss a sustainable plan for the school.”
Thomas Bennett was built by construction company BAM under a Private Finance Initiative and pays the company Â£1m per year as well as paying Â£176,000 per year to TKAT.
Mr Kelly said: “BAM should be ashamed that they are taking millions out of the school, money that taxpayers pay to educate our children not to fill the coffers of big business.”
A spokesman for BAM said: “BAM provides a range of services at Thomas Bennet Community College including building and grounds maintenance, catering, cleaning, caretaking, leisure facilities management, waste and pest control.
“Our service is efficient and value for money.”
The spokesman suggested the source of the financial pressure on Thomas Bennett came about because it was not operating at its full 1,400-place capacity.
He added: “BAM has had meetings with the college to explore if we can assist with savings.”
The NEU said its members had voted unanimously to be balloted for industrial action.