Salt can also be a reference to tears - and salt is highly irritant to eyes. So in the poem ' My parents kept me from children who were rough' the poet Stephen Spender is talking about the idea of hurt - both emotional and physical. We have already heard about the physical hurt the lower class boys can inflict - guessing at a punishment that involves pinning the narrator to the ground by kneeling on his arms and legs. We have heard about the name-calling and ambushes, yet in a quiet way the young narrator is not intimidated and would even like to be invited to join them. In a strange way, he is fascinated by the free unbridled style of the gang - so different from his own repressed home circumstances - even though their treatment probably makes him cry alone at night.
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.