I wouldn’t say it’s appropriate to “insult,” but, if it is breaking dress code, then yes, it is appropriate for a teacher to discuss it with the student privately. In my experience, most teachers do not want to deal with the dress code and have administration handle it, whether it is by calling their attention to it, or just leaving it until they catch it themselves. It is usually more difficult for a male teacher to discuss dress code with a female student, but female teachers can experience similar situations. It always depends on the situation and the specific rules at the school.
Teaching In Real Life 