To me, this poem is about a middle class (or above) boy who envies and fears the boys from the lower classes. He is intimidated by them and their behavior, but he also wishes he could be as free as they seem to be.
The line you cite is in the second stanza where the speaker is talking about the aspects of the boys that he fears. I think that "salt coarse" is a metaphor that likens the way they point at him to salt. Salt is coarse (at least rock salt is) and it can sting if you are hit with it or if it is rubbed in wounds. So the way the boys point at him is coarse (unmannerly, which is frightening to a middle class mannerly boy) and it stings in the wounds that are already in his mind (wounds that come from feeling afraid and constrained from doing as he wants).
So, I believe that "salt coarse" is a metaphor for how the boys' pointing makes him feel.