When you are a teacher, it's very important to set up a series of expectations for your students: what the rules of the classroom are, what happens if students don't follow the rules, how to achieve a high grade, how to earn yourself a low grade, how to get help when you need it. Setting expectations is important to people have a good idea of what's appropriate, which translates, to me (pardon the pun), to the culture of the classroom.
Here's an example of a negative classroom culture
In fourth grade we had a wonderful teacher who left on maternity leave, so we had a long-term sub whom I'll call Mrs. Ratchet. Mrs. Ratchet was a substitute teacher who subbed often and we all knew she wasn't always fair.
One morning I walked into the classroom and my name was on the board. (This was a bad thing, a punishment for misbehavior.) My name hadn't been on the board when I left the day before, so I was shocked. I asked Mrs. Ratchet why my name was on the board. "You know what you did!" she tossed at me as she turned away. I had no idea and neither did my friends.
To this day I remember her being so unfair, her treating me so disrespectfully, and whenever I think of it, I remember the yucky feeling I had that day. And how I wanted our regular teacher back from maternity leave because she had clear expectations. Mrs. Ratchet had changed the rules in the middle of the game!
This is an example of the kind of teacher-student hierarchy that Mrs. Ratchet practiced: