My limited subject is The power struggles in a class room. Now how can I get a thesis out of that.

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.

http://www.theapple.com/benefits/articles/8039-how-to-deal-with-power-struggles-in-the-classroom

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http://specialed.about.com/od/behavioremotiona1/a/pstruggle.htm

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.