The correct way to ask your question would be: "What defines a good student to a foreign language teacher? Only high marks (good grades)? I'm a Chinese student." To answer your question: No. Of course not! I have been teaching foreign languages in the US for some time now. The students who impress me the most are not always the ones who make the best grades (get the highest marks), but the ones who put in the most effort. I have students who don't put any effort at all, but because they are gifted at language-learning, they make good grades. I am much more pleased with a student who makes low grades, but works for it. :) Source: personal experience