i see absolutely no reason why a "teacher/student" relationship need be hierarchical or authoritarian. the idea of expertise (often expressed as "authority", a poor choice of words imo) need have no relationship with the idea of hierarchy (or power/authority). i am more skilled at building rustic structures than my friend who is more skilled at growing food. there is no hierarchy there, simply individuals sharing freely.
the question of when a child is capable of making their own autonomous decisions is a tricky and nuanced one. but the extent to which modern humans (at least in my experience) coddle and over-protect young beings is a huge problem in the growth of strong, independent-minded beings, imo. and that coddling is typically done using authority (compulsory education?). analogous to the way nanny government tries to coddle and over-protect its constituency.
the idea of "justified" hierarchical/authoritarian relations sounds like chomsky's school of thought.
literally anything can be "justified" (or "unjustified"), based on one's perspective and situation. once the analysis goes beyond that, all kinds of assumptions and generalizations come into play. such as the idea that there is some singular, common baseline against which "justification" becomes relevant/necessary (that baseline is typically defined by authority). for me that is a completely individual thing.