The priesthood is the power and authority of God. A teacher is an office in the Aaronic Priesthood. Beginning in January of the year they turn 14, worthy young men may be ordained to the office of teacher—an office they typically hold until the year they turn 16.
Teachers are organized into quorums in their wards and branches. Each quorum is presided over by a teacher who is set apart as president and who holds priesthood keys to lead the quorum (see Handbook 2: Administering the Church, 8.3.2; Doctrine and Covenants 107:86).
The bishopric is the presidency of the Aaronic Priesthood (see Doctrine and Covenants 107:15). The first counselor in the bishopric counsels with the teachers quorum and teaches them the duties of their office.
He prepares the sacrament.
He is to “watch over the church always, and be with and strengthen them” (Doctrine and Covenants 20:53). One way he does this is by serving as a ministering brother. He is assigned as a companion to a Melchizedek Priesthood holder.
He is to “see that there is no iniquity in the church, neither hardness with each other, neither lying, backbiting, nor evil speaking” (Doctrine and Covenants 20:54). This responsibility includes being a peacemaker and being an example of moral integrity and uprightness.
He is to “see that the church meet together often, and also see that all the members do their duty” (Doctrine and Covenants 20:55).
He assists the bishopric in other ways consistent with the office of a teacher.
“The Teachers Quorum,” Ensign, February 2005
Russell Wilcox, “My Teachers Quorum Is an MTC,” Ensign, October 2013