A strict teacher punishes children from time to time. However, this punishment is consistent and logical. Even if a child doesn't necessarily like a strict teacher punishing him, it will be clear why the teacher is doing so. A mean teacher, on the other hand, hands out punishments at random. The reason is that mean teachers are not interested in actually changing student behavior but expressing their own mean spirit.
A strict teacher gives the same punishment for the same transgression, every time -- or most of the time. A mean teacher, on the other hand, may punish one child for a behavior, then ignore the same behavior from another child. What's more, things that are acceptable one day for a mean teacher may not be acceptable the next day. The reason is that mean teachers base their actions on their own moods and feelings rather than on the interests of the students.
A strict teacher's class is orderly. Strict teachers have a fixed routine, clear expectations for students and are respectful of everyone in the class. A mean teacher, on the other hand, do not have a very orderly classroom. The reason is that his students will only do what they feel will keep him off their bad side, rather than what is expected of them. This creates a situation where the teacher is happy but the students have little respect for him, which creates disorder.
A strict teacher responds to her class proportionally. If they act up a little, they will receive a little punishment; if they act up a lot, they will receive a large punishment. A mean teacher, on the other hand, does not respond proportionally. Rather, she dishes out large punishments for every behavior because she can't see the nuances in managing classrooms.