The Dean Trust which runs Ashton on Mersey in Sale wants to send the pupils to Broadoak School in Partington.
Parents claim this would be "unlawful" as their children's Educational Health and Care Plan (EHC) states that they should attend Ashton on Mersey.
The trust said it was "oversubscribed".
Parents have contacted a solicitor who has written a formal pre-action letter to the school outlining their legal position.
If the school does not respond to the letter by Friday parents will issue judicial review proceedings, James Betts from Simpson Millar Solicitors said.
Deborah Keay's 11-year-old son Samuel who has autism has been told he will be one of the pupils with special needs taken by bus to Broadoak, also run by the Dean Trust.
She said the change would be very stressful and upsetting for her son.
"I wanted him to go to a mainstream school, but a mainstream school that had quite a number of years of dealing with children on the autism spectrum.
"I know and understand they are a victim of their own success but, at the end of the day, my child's school named on his EHC is Ashton on Mersey. It is not Broadoak and it is not the Dean Trust."
Mr Betts, who is representing seven of the families, said the school is legally obliged to admit those pupils under section 43 of the Children's and Families Act 2014.
"The school is still a separate legal entity and so, in the EHC, it would say Ashton on Mersey school... so that is clearly the school that they are under a legal duty to allow them to attend."
Councillor John Lamb, chair of governors at Ashton on Mersey School, said it already provided just under 8% of its school places to students with special educational needs.
"If you think about a comparable neighbouring school that same figure is under 1%."
He assured parents: "We would provide at Broadoak, as we do here, that specialised care and attention that those children deserve and need."
Trafford Council said: "Where the parent of a child with a draft educational healthcare plan expresses a preference for an academy... the council will name it in the final plan, unless, after consulting the school's governing body, one of the statutory exceptions are made out."
The Regional School Commissioner is responsible for ensuring all Academies continue to meet their statutory requirements, it added.
A group of parents whose children have special needs is threatening legal action against an academy school in Trafford after accusing it of "discrimination".