When does one finish being a student? When the resistance to being a student is finished.
Who is the teacher? The teacher is the most obedient and voluntary of all students. The teacher is not a non-student; the teacher is the first student, the best student, the most voluntary student.
So, when the student stops resisting being a student, that is the day he is a teacher—without even knowing that he is a teacher. As long as you do not like being a student, it is established that you must remain a student.