One common rude behavior is yelling at students for no good reason. For instance, if a student is a bit late to class, instead of asking why politely, the teacher just shouts at them in front of everyone. Another is publicly shaming students for their mistakes, like reading out a wrong answer in a really humiliating way.
To speak rudely meant to speak arrogantly and impolitely. This idiom is usually used to describe a person who insults, attacks, or makes trouble for no reason. People who spoke rudely often did not consider the feelings of others. Their words were sharp and could easily lead to conflict.