In the movie why doesn't scout want to come into the breakfast room on her first day of school?

I believe it is because she is wearing a dress.

Scout is a tomboy, and she never wore a dress in her life, having spent most of her childhood around boys. Now, she has to start going to school and she has to dress properly, like a good girl, which is something she is not fond of. So she is embarrassed of people seeing her in a dress which is why she doesn't want to come into the breakfast room.