If anything, it would make better sense to focus on those who struggle through lessons - the best students either are smart enough to understand their lessons and/or put sufficient effort to learn; its the strugglers who require attention. But invariably, most teachers direct their attention at the brightest. Why? I muse.
For one, it has to do with the skewed student-teacher ratios that most schools (at least in India) have. The skew causes undue effort on the teacher's attention span, so that they tend to focus their limited energies on those who are easiest to teach; it therefore, takes no leap of imagination to understand why the brightest rule - they are in sync with what is being taught most of the time, unlike the poor strugglers who gasp at the whizzing bouncers (ah! I still remember the feeling).
Another related reason, is that the best (perceived) students are invariably also the more competitive and aggressive of the lot. So in the babble of the class, the teacher's attention is more taken up by these, who end up asking the most questions and answering a bulk.
A third potential reason, is linked to how teachers get incentivized. Not every teacher is fundamentally motivated by an intent to change the world through pedagogy. For most, its a profession. A profession in which incentives are often linked to the output of the smartest student. Remember how teachers revel in pride when their student tops a school, a major examination or some day wins a Nobel! On the other hand, which teacher is ever felicitated for the poor struggler who barely managed to make it through?
To side with the teachers, one must admit the fact that teaching is a demanding occupation, requiring intense emotional (and physical) involvement and leadership skills that are no less than any other occupation. It is also amongst the most thankless. From the limited teaching I have done, I will attest as much.
So there is limited point in criticism of the teachers. But one must recognize that these behaviors do emerge, and as such might not result in the best of outcomes.
As a concluding afterthought - a personal recollection. I tend to have a quiet disposition and in most of my high-school classes, was often amongst the quietest in class. As a result, at the start of class sessions, I was hardly ever given attention by teachers, and used to struggle to catch their eye if I had a question.
Luckily for me, math and science were my strong points. After the first test or examination in these subjects, I could almost always sense a visible change in the amount of attention I received. The teacher's roving eye would sweep the class and rest on a bunch of us who had scored well.(There were even days, I thought I had perfected the art of predicting test scores in advance based on how the 'eye' swept the class!). Those days, it was often gratifying and embarassing - for along with it came the undue directional focus of questions from the teacher.
But now I look back and wonder - why, oh why, do most teachers focus their efforts on the their best students in class?