Why don't all schools use entrance exams?
Not all schools use entrance exams because they can’t all be selective about who they admit. The comprehensive system in the UK means that every child gets fair access to a secondary education regardless of their ability.
Historically, children would sit an 11+ exam, and those who passed would go to a grammar school. Those who didn’t pass the 11+ exam would go on to a secondary modern or a technical college.
In the current day, many schools are converting to having academy status, and therefore have more control over their admissions criteria.
Most schools, rather than using an entrance exam, use internal selection tests to direct students into streams and sets. Streaming is selection based on a general ability across all subjects, and setting is selecting pupils based on their ability in a particular subject. For this reason, you don't necessarily need to worry if your child doesn't secure a place at a selective school, as any comprehensive will be able to determine their strengths and weaknesses and challenge them accordingly.
For example, a pupil may be seen to be generally academically sound and have a particular aptitude for English. This pupil would probably be put in a middle band (streaming) and a high set for English, where they would encounter higher ability material (setting).
In general, the greater the demand for a school, the more difficult it will be to get a place and the more selective the admissions criteria will be. This can vary greatly from area to area based on the population and Ofsted reports of the school.