Often, the assumption underlying these stratification policies is that students' talents will develop best when students reinforce each other's interest in learning, and create an environment that is more conducive to effective teaching. Vertical stratification refers to the ways in which students progress through school as they become older. Even though the student population is differentiated into grade levels in practically all schools that participate in PISA, in some countries, all 15-year-old students attend the same grade level, while in other systems they are dispersed throughout various grade levels as a result of policies governing Ihe age of entrance into the school system and/or grade repetition.