However, Secretary of State, Nicky Morgan, has insisted that the Government has not changed its mind on selection.
Weald of Kent school in Tonbridge has been allowed open an "annexe" catering to around 450 pupils at a site in Sevenoaks. The extension is expected to open in 2017.
Clive Ireson, from the Association of Christian Teachers, says he does not disagree with grammar schools but says the difficulties are more practical.
He said: "The difficulty is the other schools in the area because they will have creamed off some of their brightest children, and then their results won't be as good probably, and then they'll be in trouble with Ofsted - so somehow you've got to square that circle."
Some have accused Nicky Morgan of skirting around the rules and say it could lead to divisions in the education system, with poorer pupils missing out.
Clive Ireson told Premier that: "There may well be legal challenges, I understand that the staff will have to work between the two schools, which is quite a way to be travelling actually, if the other school is ten miles away.
"It's thought this may well open the floodgates for other grammar schools who would like to expand as well."
He said that: "It's taken the government a long time to decide because the law says they can't create new grammar schools at the moment.
"But of course this is being declared as an expansion of a school, although it is ten miles away from the other campus."
Listen to Premier's Aaron James speak to Clive Ireson here:
