Teacher B is correct. The thesis states the main idea of the entire essay. Each paragraph should serve the purpose of supporting and/or developing the essay's thesis.
Often, the writer will develop multiple supporting points to achieve this aim. Each body paragraph will develop a different idea which supports the main idea, or thesis of the essay. These smaller ideas ideally will also have a statement within the paragraph that sums them up. Teacher A may have been thinking of the topic sentence, which should reflect the main idea of the paragraph it's in.
