Why do you have to a test enter grammar school?

You don't *have* to take a test to enter grammar school in every location. Whether or not a test is required depends on the specific school and its location. Many grammar schools, particularly in the UK, use entrance exams to select students based on academic ability. However, other grammar schools may use different selection criteria, such as teacher recommendations or school reports. In some areas and countries, the concept of "grammar school" doesn't exist, or it's used differently. So, the answer depends entirely on the specific school and its location.