שַׁאֲנַ֨ן מוֹאָ֜ב מִנְּעוּרָ֗יו וְשֹׁקֵ֥ט הוּא֙ אֶל־שְׁמָרָ֔יו וְלֹֽא־הוּרַ֤ק מִכְּלִי֙ אֶל־כֶּ֔לִי וּבַגּוֹלָ֖ה לֹ֣א הָלָ֑ךְ עַל־כֵּ֗ן עָמַ֤ד טַעְמוֹ֙ בּ֔וֹ וְרֵיח֖וֹ לֹ֥א נָמָֽר׃ (ס)
Moab hath been at ease from his youth, And he hath settled on his lees, And hath not been emptied from vessel to vessel, Neither hath he gone into captivity; Therefore his taste remaineth in him, And his scent is not changed.
Shulchan Arukh, Yoreh De'ah
A teacher of children who leaves the children and goes out, or who does other work with them, or who is lax in his teaching, he is included in the statement: "Cursed is he who does the work of Hashem deceitfully" (Yirmiyahu 48:10). Therefore, a teacher should not be appointed unless he is G-d fearing, quick to read, and precise in teaching. Rema: And a teacher should not stay up too late at night, so that he will not be lazy during the day to teach. And similarly, he should not fast, or abstain from food and drink, or eat and drink excessively, for all these things cause him not to be able to teach properly. And anyone who deviates, his hand is on the bottom and he is dismissed. Mordechai, Perek Ha'umanin from the Yerushalmi (The Jerusalem Talmud). And the rest of the laws of a teacher, see in Choshen Hamishpat Siman 334 and 335.