The school has a duty of care as soon as a pupil has been registered in writing (from the age of 3). When a pupil is registered at a school and parents register at another school, the school where the child was most recently registered has the duty of care.
Parents of students who need extra support are sometimes referred from one school to another without a school admitting their child because there is no written application yet. This is undesirable and also has consequences. Read on the website of the education inspectorate what the school's duty of care entails
