Student selection and sorting may also create stereotypes and stigmas that could eventually affect student engagement and learning. Vertical stratification refers to the ways in which students progress through school as they become older, horizontal stratification refers to differences in instruction within a grade or education level. School systems determine which specific programmes to offer (vocational or academic, for example), the age at which students are admitted into these programmes, and the extent to which students' academic records are used to select students for their schools.