What would the schoolmaster say to the father when the child was ready?

The tutor would say to the father, "Master, this young man is now ready to come in and work right with you in your business. He has everything he needs to know. He has been well tutored, and he is well prepared."

Les explains that the schoolmaster Paul refers to is not in the Jewish economy but in the Greek and Roman world. The Greeks and Romans would hire a slave, a servant, who was instructed and placed under the father's demands to raise, tutor, and teach the child. At some designated point in the future—whether the boy was 16, 18, or what today would be 21 or whatever—the tutor could tell the father that the young man was ready to come in and work with him in his business, having everything he needed to know and being well tutored and prepared.

Galatians 3:25

But after that faith is come, we are no longer under a schoolmaster.

When did the schoolmaster's role end in Greek and Roman practice?

In Greek and Roman practice, the schoolmaster was a hired slave or servant who trained a child until a designated future point, perhaps age 16, 18, or what today would be about 21, when the tutor could report that the young man was ready to work with his master in business.