Our tutor unto Christ (παιδαγωγος υμων εις Χριστον).
See
That we might be justified by faith (ινα εκ πιστεως δικαιωθωμεν). This is the ultimate purpose of the law as paedagogue.
Now that faith is come (ελθουσης της πιστεως). Genitive absolute, "the faith (the time of the faith spoken of in verse Galatians 3:23) having come."
Under a tutor (υπο παιδαγωγον). The pedagogue is dismissed. We are in the school of the Master.