Here is the Act as originally passed, it was amended in 1996 and subsequently. I beleive that makes the actions of the head unlawful in the case mentioned.
47Abolition of corporal punishment
(1)Where, in any proceedings, it is shown that corporal punishment has been given to a pupil by or on the authority of a member of the staff, giving the punishment cannot be justified on the ground that it was done in pursuance of a right exercisable by the member of the staff by virtue of his position as such.