I suggest you start by brainstorming your ideas about these power struggles. Jot down brief notes as you think of them. Do you see a pattern? Do you see where you might go with this topic?
A lot has been written about how teachers can diffuse these power struggles.
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Perhaps your thesis could be a way a teacher could avoid most power struggles.
Perhaps your thesis could be some of the reasons students challenge teachers.
Or maybe you have some other ideas.