Disclaimer: Such a topic is highly subjective. This comment here is to simply offer a different take on the aforementioned incident. To do so, I must reply to OP, making it seem I am addressing him directly but I am in fact responding to the incident in the abstract. OP, I wish to in no way diminish your pain. My only goal here is to use the experience that you have shared to encourage introspection and self-examination at an individual level.
If the student's academic prowess was well established before this incident and the parent had already come to accept it, then there are grounds to claim that this is abuse. If the student's academic prowesses were well established before this incident but the parent had not come to accept it, well, it was unfortunate. Hopefully the parent eventually came to accept the child for what he is. If this incident was the first time that the student ever failed, then there is some justification* in the actions taken by the parent.
I suppose in the reasoning of the parent, the hair-style indicated shifting attention in the child (who at this age would have been in throes of puberty) and the solution was to punish the child through shame. It is the shame that the child, grown up now, remembers, the shame of having to face the peer group to whom the association betwen a change in hair-style and academic failure would have been immediately apparent. (Any other time, it would have just been a change in hair-style.)
*YMMV depending on class, family situation, importance of education to the family, etc.,