Is the testimony of teachers not accepted Islamically, and what is the reason for that?

There is no basis for not accepting the testimony of a children's teacher. Teaching is an honor that elevates the status of its practitioner, and the teacher of good is blessed by Allah and His angels, even by ants and whales. Ibn Hajar al-Haytami mentioned that what was transmitted from "Al-Khulasah" regarding this matter is incorrect, and that if the aforementioned statement is authentically attributed to a scholar, it must be interpreted as referring to a teacher who displayed signs of ignorance, immorality, or insanity.