We have different opinions about this.
The first opinion states that teachers should teach their students in national or official languages. I am taking an example for our country, Indonesia. You need to know that Indonesia is an archipelagic country. Each province has different variants of the mother tongue. One of the mother tongue that is often used by the people of Indonesia is the local language.
Indonesian is the national language. Each region, although having various mother tongues, is obliged to use the national language as the language of communication in formal life. For example school. Schools are required to use the national language during the teaching and learning process. The teacher should explain the subject matter in the national language.
The national language is the language of Indonesian nation identity. So, by familiarizing the use of Indonesian language in the school environment, children will also be familiar in using the Indonesian language properly and correctly.
What about the second view?
The second opinion states that there is nothing wrong for teachers to use the mother tongue in the classroom. Teachers in the countryside are sometimes difficult to explain the subject matter to their students in the national language. These children better understand the subject matter with their mother tongue. However, the use of the mother tongue is limited only in the explanation of course material, while the textbooks remain delivered in the Indonesian language.
Currently the incessant socialization of mother tongue preservation. One of the preservation of the mother tongue that can be applied is in the school environment and daily interaction.
So, in my opinion, teachers are allowed to teach in the mother tongue in the classroom, but only limited to explain the subject matter that is difficult to absorb by some students who still use their mother tongue. I use mother tongue to analogy and explain a subject matter.
In this way, no student feels neglected, although some of my students do not understand the mother tongue or the local language.
Some people may assume that the mother tongue is an old-fashioned and abusive language. But in my opinion, the mother tongue is a pure original language. In the mother tongue contains ethical values and (even) high aesthetics. I am proud to use my mother tongue. And my mother tongue is Aceh Language.