A good student is someone who has the aptitude and the desire to learn his field of study. So he does everything to learn the subject because he really wants it.
Most of the students in IITs say that their coaching teachers were better than those in IIT. But their completely remove their desire from the equation which skews their perception of teachers in IIT. The students in coaching classes are very interested in clearing JEE so they do everything to learn. Even if the teacher is bad they learn those subjects. They don't go on complaining about the teachers and don't study. But in IITs it is very common because the students are not interested. They keep on blaming the teachers for making their life miserable. It is easy way out. They don't take responsibility of their lack of interest. In Hindi there is a saying that you can take a horse to the pond but you cannot make him drink the water. There are good teachers in IITs but the students are not interested. So they lose their interest in teaching because there is no tangible reward for doing well and the students don't care.
So an interested students is a good student. Simple.