Moreton Hall will have boys in years seven and eight from September 2021, before they leave to join other senior schools.
The Oswestry school will be taking boys from its own prep school as well as children from outside.
The school says the move is part of its strategic development plan to blend the best aspects of the prep and senior schools.
John Bond will arrive at the school as head of prep at Easter next year.
A school spokeswoman said: "This change offers the best of all worlds - our all-girls provision is retained in our more senior years, and our boys do not have to prematurely leave a school where they are settled and happy.
"The Boarding House remains at the centre of academic and pastoral life for day and boarding pupils and our personal tutoring system continues. Lessons can take place in their usual subject-specific venues around the school and all pupils, boys and girls, can access the full range of senior school co-curricular activities.
"We firmly believe that our proposals are educationally sound, in the interests of current and future Moretonians, and help to cement the reputation of Moreton Hall as a forward-looking school for girls to age 18 and boys to age 13."