If your child has an Education Health and Care Plan (EHCP), the discussion and any planning for transfer to secondary school is carried out at the Year 5 annual review.
It is important, therefore, that you visit any schools you are considering for your child before this review because you will be asked to name your preferred school. This will then be sent to the Local Authority with the other review documentation for consideration by the SEN Department.
Any amendments to the EHCP will be made in Year 6. You will receive a form to fill in stating the school you want named in Section I, along with your reasons for your choice.
The amended EHCP is generally issued by 15th February in the year of transfer.
This guide is aimed at those with Autism Spectrum conditions but a lot of the information can be applied to any child with a disability / additional needs: http://www3.hants.gov.uk/sen-transition-secondary-asd-parents.pdf