The poem begins on a joyful note. The title "The School Boy," combined with the speaker's childlike exuberance, signals that this is a dramatic monologue: a poem voiced by a character separate from the poet. In other words, the speaker here is a schoolboy—an unnamed, average, school-aged child in William Blake's 18th-century England—not Blake himself.
Like many children, this schoolboy is a fan of summer. He declares that he "love[s] to rise in a summer morn"—in other words, he loves waking up on summer mornings. He delights in hearing "the birds sing on every tree" (a sign that he lives in the countryside, where there are lots of trees around). As he enthuses about the season, alliteration adds a gentle musicality to his language, so that the lines themselves seem to "sing":
The distant huntsman winds his horn,
And the skylark sings with me:
O what sweet company!
Those soft /h/ and /s/ sounds make the phrases themselves sound "sweet" and euphonious. The "huntsman" is a hunter of foxes or other game, "wind[ing]"—that is, blowing—his "horn" as a signal to his dogs or fellow hunters. Again, this detail evokes a rural setting—a pastoral landscape that the boy would like to be out enjoying, just as the hunter is. Meanwhile, a "skylark" seems to "sing[]" along with the boy in the trees or sky above. (Larks are famously active during the early morning hours.)
Together, the huntsman and the lark seem to make for "sweet company," or wonderful companionship. But there's a touch of irony here, since the hunter is "distant" and the bird isn't human; in other words, the boy doesn't actually have any friends around. He may, in fact, be a little desperate for some "company." And as the rest of the poem makes clear, his joy in the morning is only too brief. He can't go out and play on this "summer morn," nor will he be "sing[ing]" for most of the day. He has to go to school instead.