Wherefore the law was our schoolmaster to bring us unto Christ, that we might be justified by faith. --Galatians 3:25
def n a boy's guardian or tutor, a slave who had charge of the life and morals of the boys of a family, not strictly a teacher
The schoolmaster in the Roman home was not so much a teacher as a disciplinarian. This person, usually a slave, was placed by the father of the child over the moral conduct of the child until such time came that the father decided the child had reached maturity. Until the father thought the boy was mature he was kept under the constant surveillance of the schoolmaster.
Here in Galatians the Law is likened to a schoolmaster. The Law was given to show man that his sin was an transgression against God and man was in need of a savior. Thus, when God sent forth His Son, Jesus Christ, to be the needed Savior man would recognize his need turn to Christ. In verse 22 of the same chapter the Bible states, "scripture hath concluded all under sin." All pathways to God are shut up except the path of the promised seed... the Savior Jesus.