Prophet Musa عليه السلام humbled himself and sought permission from Khadir to learn from him:
قَالَ لَهُ مُوسَى هَلْ أَتَّبِعُكَ عَلَى أَن تُعَلِّمَنِ مِمَّا عُلِّمْتَ رُشْدًا
“Musa said to him, “May I follow you on the understanding that you will impart to me something of that consciousness of what is right that has been imparted to you?” [Al Kahf 18: 66]
So having sought permission, what was Khadir`s response?

He said:
قَالَ إِنَّكَ لَن تَسْتَطِيعَ مَعِيَ صَبْرًا
“Behold, you will never be able to have patience with me” [Al Kahf 18: 67]
It is interesting to note that Khadir used the word “لَن”, not “لا” – which carries much more certainty within itself. He continued:
وَكَيْفَ تَصْبِرُ عَلَى مَا لَمْ تُحِطْ بِهِ خُبْرًا
“For how could you be patient about something that you cannot comprehend within the compass of experience?” [Al Kahf 18: 68]

Khadir explained that the reason he believed that Prophet Musa عليه السلام “would never” be able to be patient enough to learn from him, was because he did not possess enough life experience to keep silent. This is also true, that younger people, or those who have not had vast and rich life experiences, tend to be more naive and superficial when judging the reality of events, tending to take them at face value instead of being able to grasp their deeper, hidden truths.

What we also learn is that if the teacher perceives that the student doesn’t have the qualities of studying under him, than he has the right to refuse admission to him. The teacher should also not force the student to remain with him. Sometimes the student doesn’t have the ability and sometimes certain students might not have the qualities, then they should be allowed to leave and seek knowledge elsewhere. It is very possible that from a different teacher and with different methods they could learn better.

It is also important for the teacher to know his student. Khadir knew his student well. Another etiquette of the teacher with the students is being able to speak to the student according to the student’s level. Khadir is saying that he has knowledge of Allah’s wisdom that Musa عليه السلام did not have, but he doesn’t humiliate him.

قَالَ سَتَجِدُنِي إِن شَاء اللَّهُ صَابِرًا وَلَا أَعْصِي لَكَ أَمْرًا
“Replied [Musa]: “You will find me patient, if Allah so wills; and I shall not disobey you in anything!” [Al Kahf 18: 69]
Prophet Musa insisted that he’d practice patience إِن شَاء اللَّهُ (by the will of Allah).

The student once granted admission to the learning institution, should then endeavour to try his best and apply his mind fully to his studies.

Khadir then permitted him to come along to learn from him, on the condition that Musa not ask him any questions about anything that he saw him doing, until he spoke of it himself.
قَالَ فَإِنِ اتَّبَعْتَنِي فَلَا تَسْأَلْنِي عَن شَيْءٍ حَتَّى أُحْدِثَ لَكَ مِنْهُ ذِكْرًا
“Khadir said: Well, then, if you are to follow me, do not question me about anything [that I may do] until I myself give you an account thereof.” [Al Kahf 18: 70]
What we also learn is that the teacher or institution has the full right to set their own rules which all students must abide by.