A: I personally would not draw the conclusion that the boy is disadvantaged so much as the little runt that the bigger boys pick on. Note in lines 12-14 the reference to being taught in a ruder style, referring to being beaten up. Note how far from home to school he has to walk and endure the indiginities heaped upon him by these other boys.
Line 2's "Sanctum Sanctorum" tells us that he is happy to be here, it is a refuge. He is arrested not because of shock, but out of wonder, at being in such a reverent place. The library is for him like a church. Look up the def. of Sanctum Sanctorum.
Check the alternate definitions of line 5's "stealing." Consider especially the ideas of hardening ones nerves, of bracing one's self to endure some coming problem. Consider also the idea of moving about in hidden way, of seeking to avoid detection.
In line 6 we learn he is in "his own place." Connect this back to Sanctum Sanctorum. Also note that he is enchanted with himself when in the library. Could it be that only here he feels alive? That he can find value for himself, his thoughts?
LIne 7: Do you have a friend who plays the trumpet? If so ask them about "tounging." It's one of the basic techniques that allows the various notes to sound. Requires very rapid movements. His heart must be beating very fast. He is having an adrenilin rush. He is excited to be here. The kind of heart pounding thrill one gets when one is looking forward to some great event.
Lines 8 & 9 merely reinforce that he is safe.
Lines 10 & 11 give us a hint that his love of books and worlds therein set him apart from the other boys as being very different, different as if he were blind, etc.
The poem it seems to me then summarizes how some boys find refuge in the imagination, in words and the worlds they create, and in the books that hold these words. No longer trapped in their own bad life, the boys can be transported to worlds far happier, more just, etc. by an author's writings. This ability to escape through writing is what drives most authors, who often start as boys finding refuge in a library.