very often when a child tells you something has gone wrong, you don't get the full story. This may be because the child sees things very literally, or one dimensionally, or can only see their perspective, or misheard, or misunderstood, or forgot or plain lied, possibly by omission.
If you go directly to the head, the most you can do is voice concerns. the head doesn't know the whole story, can't really answer your questions. it just doesn't make any sense. You go to the teacher. You discuss with them. If you're not happy with the teacher's response or with the outcome, or dc says problem is still ongoing, then you go further.
Going straight to the head implies you don't trust the teacher, you assume they're wrong, you are not able to deal with conflicts/situations, it's immediately accusing someone without giving them the opportunity to explain, and frankly leaves you open to potentially looking foolish if the dc has got the wrong end of the stick or it is in some way a misunderstanding. And it makes what could be a simple chat with a simple explanation a time consuming process which will potentially lead to bad feelings.
Over the years, there may well be many situations you need clarified. DO people really run straight to the head every time?