These three young Junior High Schoolgirls are Miko, or shrine maidens, and are taking part in the procession of the mikoshi at a matsuri. Before the procession they had performed Miko-mai, a sacred dance in the shrine.
These miko are volunteers, and just train for this performance. At many of the larger shrines that have full-time staff you may see older Miko, working in the shrine shop or office. Nowadays the only requirement to be a miko is being unmarried, though in earlier times it was necessary to be a virgin. Before that shrine miko were pre-pubescent girls who retired at the onset of puberty. In Izumo the tradition was different, and miko remained miko all their lives and passed on the function to their daughters. Nowadays many miko are daughters of priests.
Miko also assist the priests with ceremonial duties as in the photo below where a miko is performing Oharae, a purification ritual. There are a few female priests in Shinto, but not many.
Another type of miko no longer existing were the wives of Shugenja. The miko would go into a trance under the control of her yamabushi husband. Miki-san, the founder of the Tenrikyo religion was functioning as a stand-in miko when she began to have trance. Shamanesses and priestesses were probably the norm in ancient times before the influence of Buddhism.