In 1969, the government abolished entrance examinations for middle school students, replacing it with a system whereby primary school students within the same district are selected for middle schools by a lottery system. This has the effect of equalizing the quality of students from school to school, though schools in areas where students come from more privileged backgrounds still tend to outperform schools in poorer areas. Until recently most middle schools have been same-sex, though in the past decade most new middle schools have been mixed, and some previously same-sex schools have converted to mixed as well. There are some schools that converted to same-sex because parents pressured the schools because they think that their children can study better in single-sex education.