The Education Authority (EA) plans to close the Lurgan campus of Craigavon Senior High School (CSHS).
It would mean all of the school's 618 pupils attending the Portadown campus from September 2020.
The EA's case for change was set out at a meeting in the school on Monday night and is now subject to consultation.
A group representing some parents in Lurgan is opposed to the proposal and has accused the EA of a "box ticking exercise".
A number of controlled schools in the Craigavon area operate the Dickson plan, where pupils attend an all-ability junior high school from ages 11 to 14 then transfer to CSHS, which is non-selective, or one of two local grammar schools - Lurgan College and Portadown College.
Since opening in 1995, CSHS has operated a split site, on two separate campuses five miles apart, one in Portadown and one in Lurgan.
The Portadown site is larger and had 444 pupils in September 2018, while 174 pupils were based in Lurgan.
The Lurgan campus was the subject of a critical review by the EA in 2016 which identified a number of problems with the site.
It said that the personal safety of pupils was at risk due to the condition of the school building.
"In relation to the personal safety of students, this campus is not fit for purpose," it said.
It found that pupils at the school had to mix with members of the public and Southern Regional College (SRC) students when using some classrooms, as SRC also shared part of the site.
The 2016 review also said that, because there were no outdoor PE facilities on the Lurgan site, pupils had to walk a mile though the town in their PE kit to use other facilities leaving them "stressed, frightened and vulnerable".
The school also has an ongoing budget deficit of more than £1m, partly due to operating on a split site.
The EA's case for change said CSHS's GCSE results were below the level of similar non-selective schools.
It said that pupils would have access to a wider range of courses and extra-curricular activities if the school was based only in Portadown.
'Fresh ideas needed'
However, the Education Equality for Lurgan group, which represents some parents, accused the EA of a "box ticking exercise".
"The outcome has been stacked against Lurgan children," they said.
"What we need in Lurgan is fresh ideas that are going to deliver for our children, not more of the same failure only miles further from home."
The group has also expressed concern that pupils in Lurgan, aged 14 and over, would be left without the option of attending a controlled school in the town.
They have put forward an alternative plan for Lurgan Junior High School to change from a school for 11 to 14-year-olds to one for 11 to 16-year-olds, and said this "would provide a sustainable school in the heart of the community".
'No longer fit for purpose'
However, the EA said that option would undermine the Dickson plan and go against a statement made by the former Education Minister Peter Weir in 2016 in which he said it should not be removed.
The EA also said that it was working on a wider plan to introduce a sixth form to CSHS within the next three to five years and would bid to the Department of Education for funding for a new school building.
"The Education Authority is committed to providing a viable and sustainable Craigavon Senior High School that delivers an excellent education for its pupils," a spokesperson said.
"The school, which operates across two campuses, currently provides education for 14 to 16-year-olds (Key Stage 4) in the Craigavon area.
"One of the campuses - in Lurgan - is no longer fit for purpose and struggles to provide an appropriate curriculum experience for its pupils."
They also said the consultation would be open to the views of parents, staff, governors and pupils of schools in the area.