Deletions from Admission Register
8.—(1) The following are prescribed as the grounds on which the name of a pupil of compulsory
school age shall be deleted from the admission register—
(a) where the pupil is registered at the school in accordance with the requirements of a school
attendance order, that another school is substituted by the local education authority for
that named in the order or the order is revoked by the local education authority on the
ground that arrangements have been made for the child to receive efficient full-time
education suitable to his age, ability and aptitude otherwise than at school;

For more on deregistration, visit the education otherwise website. There is also a sample deregistration letter here.