Paul is saying that the believing Christian need not keep the ceremonial laws, the shadow sabbaths, because they find fulfillment in faith in Jesus Christ, the true passover lamb. The schoolmaster was the yearly ceremonial sabbaths associated with the Hebrew Temple, which taught the plan of salvation through a coming Messiah, the passover lamb of God. Now that salvation through Jesus of Nazareth is clear to the Gentile church, there is no further need of the schoolmaster (the earthly Temple, its ceremonies and shadow sabbaths) because that lesson has been learned by those with faith. However, nowhere does the Bible indicate that one of the Ten Commandments has been abrogated, or that the disciples or Jesus abandoned the weekly Seventh-day Sabbath for Sunday keeping. Indeed the Bible makes clear, that in the future all believers will keep the true Seventh-day Sabbath-
Isa 66:22 For as the new heavens and the new earth, which I will make, shall remain before me, saith the LORD, so shall your seed and your name remain.
Isa 66:23 And it shall come to pass, that from one new moon to another, and from one sabbath to another, shall all flesh come to worship before me, saith the LORD.