Is it permissible for an Islamic teacher who has students (males & females) for him to inform his students that he does not want any advises?

It could be that what led this teacher to say such a thing is the misconduct of some students who advised him in an inappropriate way, and the like. Hence, let the students be heedful of this and must know that the teacher should be respected and holds an important status, so they should be polite with him when speaking to him.

If they want to advise him, then let it be in a manner that does not decrease his respect and esteem.

As far as the teacher is concerned, we advise him to accept advice from whoever offers him advice, because a Muslim must accept advice from anyone who advices him, whether old or young. This is because rejecting the truth from whoever brings it is arrogance, as the Prophet salla Allah alaih wasallam said: “Pride is rejecting the truth (out of self-conceit) and disdaining people.” [Muslim]

One of the characteristics of the believers is that they advise each other, and one of the characteristics of the hypocrites is that they do not provide advice and they deceive; Allah Says (what means): {The believing men and believing women are allies of one another. They enjoin what is right and forbid what is wrong.} [Quran 9:71]

Also, exhorting others to follow truth and exhorting them to observe patience are among the reasons of salvation as Allah Says (what means): {by time. Indeed, mankind is in loss. Except those who believed and did righteous deeds, and exhorted each other to follow truth, and exhorted each other to observe patience.} [Quran 103:1-3]

Therefore, this teacher must accept the advice of whoever advises him, and should not forbid the students from advising him. Nonetheless, if the students deem appropriate to advise him, they should be polite with him and speak to him in a manner that suits his status as a teacher.