I think that the ideal should be that no child should be left behind. The problem is, I am not a god. I am a human being. So if I have the following situation
Student 1) Doesn't even know the alphabet.
Student 2) Can read and understand native speaker 6th grade level text books.
If I try to teach 1 the alphabet, 2 will get bored. If I try to teach native speaker 6th grade level English to student 2, 1 will get left behind. If I could bilocate my voice and teach the alphabet, and 6th grade level English at the same time, I would be a perfect teacher. Also, if I could put everything I know about English onto a memory chip, and have the student directly upload that information into his brain, then teaching him English would take seconds instead of years.
On top of that, I have limitations that many other humans don't have. Since I studied mathematics in college, and was interested in "geeky" things, my social skills suck and probably always will suck. This effects my teaching style in that I am not "exciting", but sometimes it's easier for me to come up with ways of explaining certain kinds of things.
So, should students be left behind? Probably not. But the reality is, some students will get left behind because of a variety of reasons.