> you aren't even aware of the bottom percentile until the first exam is graded. The first exam that students in test-prep schools take is before classes start

> you aren't even aware of the bottom percentile until the first exam is graded. The first exam that students in test-prep schools take is before classes start, usually a mock university test that can be used precisely to assess your current level and to gather information about the class overall. You don't need to know who are the worst performers. But if you know that your class of 250 has an uneven distribution of aptitude, you'd try to break the class apart.