The introduction of such systems began in Leicestershire in 1957. West Yorkshire followed in 1963, but the system has gradually been withdrawn in some areas since the introduction of the National Curriculum. This is because of the nature of the curriculum, which is divided into Key Stages which do not fully align with the three-tier system. In some areas (such as Leicestershire) where this type of school is in use, the middle-tier schools are known as High Schools; in other areas it is the upper tier schools that are called High Schools, especially if they do not include Years 12 and 13.