Each LEA will be slightly different but this is for my local area
If the total number of applications for admission to a school, including second and third preferences, exceeds the school's Published Admissions Number (PAN), the following order of priority is used to allocate the available places, (if an applicant meets the admissions criteria of more than one preference then they will be offered a place at the school ranked highest):
(a) Relevant Looked After Children
(b) Pupils living within the catchment area of the school;
(c) Pupils who have an elder sibling in attendance at the school (or, for Infants schools, at the affiliated Junior school) and who will still be attending the school at the proposed admission date; (for admission purposes, a sibling is a child who lives at the same address and who is the brother/sister, half-brother/sister (children who share one common parent), step brother/step sister where two children are related by marriage. This definition also includes adopted or fostered children living at the same address).
(d) Pupil who satisfy both of the following tests:
Test 1: they are distinguished from the great majority of applicants either on medical grounds or by other exceptional circumstances.
Medical grounds must be supported by a medical report, (obtained by the parents). It must clearly justify, for health reasons only, why it is better for the child to attend the preferred school rather than any other school. Exceptional circumstances must relate to the choice of school, i.e. the circumstances of the child, not the economic or social circumstances of the parent. They should be supported by a professional report, e.g. social worker, justifying why it is better for the child to attend the preferred school rather than any other school.
Test 2: They would suffer hardship if they were unable to attend the school
(e) Pupils whose parents are regular attendees of the Church of England; or of a church in communion with the Church of England; or of a church which is affiliated to the Council of Churches for Great Britain and Northern Ireland or the Evangelical Alliance. Evidence of such attendance will be required in the form of a letter from a minister of the Churches concerned.
(This criterion can only be used by prior agreement with the Local Admissions Forum, which can consider its application to Voluntary Controlled Primary Schools. Use of the criterion will be listed against the school's entry in the C&LL document Information for Parents - Primary Schools).
(f) Other pupils arranged in order of priority according to how near their home addresses are to the school by the shortest walking route as measured by the Lifelong Learning Directorate's Geographical Information System.
Parents should note that in accordance with legislation, children who have a statutory statement of special educational need that names a particular school will be admitted to that school prior to any other places being allocated.
Relevant looked after children means children who are looked after by a local authority in accordance with section 22 of the Children Act 1989(b) and who (a) are looked after at the time an application is made and (b) in relation to whom the local authority has confirmed that the children will still be looked after at the time when the child will be admitted to school.
Where it is not possible to accommodate all pupils within a particular group the Education Service will allocate the available places in accordance with the remaining criteria. For example, if there are insufficient places for all catchment area pupils, places will be offered to those pupils who live in the catchment area and also satisfy criteria (c), then those who live in the catchment area and satisfy criteria (d) and so on.
All applications are considered against the published criteria with no priority awarded for early application.