You must get consent from the parent, guardian or carer of a child or young person up to the age of 18 (this is called `parental consent'). As most children attend primary and secondary schools for five years, it seems sensible for you to get consent for the whole five-year period. We suggest you send a consent form to parents with the school registration pack, to cover the period that their children will spend at that particular school.
Although this usually means that you won't have to renew parental consent until a child changes schools or starts at sixth form, you will have to be careful to record any changed circumstances. This will be easier if you keep photographs and signed consent forms together.
Source: here