Plato, Laws 810a

γεγονόσιν ἄρχεσθαι μέτριος ὁ χρόνος, ἐμμεῖναι δὲ ἕτερα τρία. καὶ μήτε πλείω τούτων μήτʼ ἐλάττω πατρὶ μηδʼ αὐτῷ, φιλομαθοῦντι μηδὲ μισοῦντι, περὶ ταῦτα ἐξέστω μείζω μηδὲ ἐλάττω διατριβὴν ποιεῖσθαι παράνομον· ὁ δὲ μὴ πειθόμενος ἄτιμος τῶν παιδείων ἔστω τιμῶν, ἃς ὀλίγον ὕστερον ῥητέον. μανθάνειν δὲ ἐν τούτοις τοῖς χρόνοις δὴ τί ποτε δεῖ τοὺς νέους καὶ διδάσκειν αὖ τοὺς διδασκάλους, τοῦτο αὐτὸ πρῶτον

and for lyre-playing, he should begin at thirteen and continue at it for three years. And whether he likes or dislikes the study, neither the child nor his father shall be permitted either to cut short or to prolong the years of study contrary to the law; and anyone who disobeys shall be disqualified for the school honors which we shall mention presently. And, during these periods, what are the subjects which the children must learn and the teachers teach—this you yourself must learn first.