Powys council wants to build a new English-medium school to replace those in Gwernyfed and Brecon by 2019/20.
It is hoped the plan will cut costs and tackle the issue of surplus places.
But many are concerned about the impact closure will have on the community, children's education and the distance some will have to travel to get to the next nearest school.
Some have even suggested this move will mean students who could attend Gwernyfed in future will be forced to go to school over the border in Hereford.
Ian Charlesworth, chairman of governors at the 450-pupil Gwernyfed High School, said: "If the school closes, it's so deeply rooted in the local community, we fear for the future of the community.
"People are frightened what it might mean once this place isn't here providing the sports facilities and the drama studio, on top of the children having to moving elsewhere to socialise."
Powys council said it had to make these changes because of a surplus in school places and to ensure better funding for the future of secondary education in south Powys.
Councillor Arwel Jones, the authority's education spokesman, said: "Any school closure is a worry for parents in the community.
"But there's around 1,500 less pupils in secondary education in Powys compared to six years ago.
"Obviously the funding to schools is linked to this and therefore the choice of subjects available.
"There is also room for improvement in standards, so the idea is to have a concentrated mass of students in one school to attract the best teachers."
The public consultation on the proposed changes ends on 22 May.
Almost 800 people have attended a public meeting on the future of secondary education in south Powys.