Rambam, Mishneh Torah, The Laws of Torah Study 2:3

An elementary teacher who leaves the children and goes out, or does other work with them, or teaches them sluggishly, falls under the category referred to by Jeremiah: “Cursed be one who does the work of the Lord with slackness” (Jeremiah 48:10). It is improper, therefore, to engage a teacher unless they are a person of reverence and skilled in reading and in grammar. [Translation by CAJE. Edited for gender neutrality]