The entry requirements of schools vary enormously. To begin with it is worthwhile to consider that they are age related and may have strict criteria for start and finish. For other schools it is possible to begin a course at a number of entry points along the way.
Testing can involve many subjects (Common Entrance) or testing in Maths and English, and possibly non-verbal reasoning.
In the UK parents have various reasons for when they start children in fee paying schools, while international children will start later in general as leaving home at a very young age is an issue. Language skills will always be an issue for international children unless they have been in an international school in their own country. As a rough guideline arriving later to take a GCSE Course (2 years age 15 & 16) will require IELTS 4.5. A Levels would require 5.5 IELTS for a two year course.
Preparation for entry begins with research into the type of school and entry requirements. A broad outline is as follows:
11+ entry to selective day schools and prep schools. Exams taken in January.
13+ Prep school entry – scholarships in May exams in June.
International students require language testing and then preparation.