School District 21

District 21 (第二一学区) is the westernmost District of Academy City. It is bordered to the east by District 9, to the north by  District 17, to the south by  District 13, and to the west by Western Tokyo.

The district's primary use is for water storage, and in astronomical observation facilities.

Principles
Unlike District 23, which is flat due to its many runways,  District 21 is a mountainous area. The mountainous area holds many dams that act as the city's water source. The water is distributed to the whole of Academy City using pipes, from uses such as beverage service to industrial use. The stars in the night sky can be seen more clearly in the district than the rest in the city. Perhaps because of this, the district contains several astronomical observation facilities. The area is also the location of an experimental geothermal power station situated on the mountain's peak, and a large-scale meteorological observatory, which is also the base for the disaster recovery group Useful Spider.

The district also keeps wilds animals due to its mountainous area.

Honey Queen Arc
Between August 5th and 6th, Item rented a cottage in the mountains of the district as a temporary base during the mission against Honey Queen, after the initial airship reservation proved unpopular in the face of an approaching typhoon.