This is, as you, know, a very powerful poem and the tone of despair is clearly indicated. These students are 'far from gusty waves' - land locked and seemingly deprived of an understanding of the elements. They are 'like rootless weeds': without foundation and nurture and we see them as sickly as indicated by their 'pallor'.
The poet then hones in on individual children to show us more closely the despair of their situation-
The tall girl with her weighed-down head. The paper-
seeming boy, with rat's eyes.
The girl is clearly exhausted, or bored, and has lost contact with the teacher. The boy is thin, pale, and compared to vermin.
The stunted, unlucky heir
Of twisted bones, reciting a father's gnarled disease,
His lesson from his desk.
This child exemplifies the poverty and tragedy of the child being the product of a poor environment and poorer health. It is in the last child that there may be hope, in that he takes his mind elsewhere, and perhaps has scope beyond the slum-
At back of the dim class
One unnoted, sweet and young. His eyes live in a dream,
Of squirrel's game, in the tree room, other than this.
thank you :)