According to most educators' codes of ethics, name-calling of students is considered unprofessional at least, and a terminable offense at the worst, meaning it can get a teacher fired. However, legally speaking, a teacher can "label" a student in certain ways that fall outside the realm of religion, race, lifestyle, or socioeconomic status. As an example, if a student comes into class with their pants too low and the teacher calls the child a "plumber," referring to the pants' exposure of the buttocks, then that label is legally defensible, although inappropriate. However, if the teacher in question labels a female student a "slut" for wearing clothing that exposes too much anatomy, that incident then falls into the realm of sexual harassment or even slander. This question also hinges upon state laws governing educator standards. You may wish to check your local laws regarding this matter.