An instructor should aim to train every student up to their level and foster the skills necessary to go further. This approach benefits the teacher as well, because a skilled student offers better partner practice and a greater challenge.
It also reflects their ability to teach. A student is not defined by their teacher. They can exceed the teacher.
Consider people such as Einstein or Mozart. Clearly Einstein exceeded the sum of his teacher's expertise; he is considered to be one of the most powerful thinkers of all time... What was his physics teacher called?
Every good teacher wants the student to exceed their own limitations.