Theory:

I love to rise in a summer morn,
When the birds sing on every tree;
The distant huntsman winds his horn,
And the skylark sings with me.
O! what sweet company.
Explanation:
 
As the title of the poem "The School Boy" suggests, the speaker of the poem is a young school-going boy. The poem begins with the speaker stating that he loves waking up on a summer morning. ‘The birds that are found singing on every tree’, ‘the sound of the horn from a hunter in the distance’, and ‘the skylark that can be heard singing along with the speaker’ make the summer morning special to him. He enjoys the morning so much for what had accompanied it, and hence, calls it a "sweet company".
 
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A huntsman winds his horn

It is interesting to note how the poet is visualizing a concert in the summer morning. The stanza begins with the music from the birds. The birds are occupying every tree as if taking their position like the musicians in an orchestra or concert. The music also includes the sound of a horn let out by a hunter hunting in the distance. The speaker is so happy that he joins the music; he sings, and to his delight, Skylark, a bird known for its songs, sings with him. The morning is so pleasant, and the speaker immerses himself in nature's concert.
 
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The birds sing on every tree

Hence, through the first stanza, the speaker expresses his delight in the beauty of summer morning. One should remember that the summer that is referred to in the poem denotes the summer experienced in England. Summer, which begins around June and lasts through August, is not as hot as the summer experienced in India. Moreover, the season often signifies life. It is also an ideal time when various sports and recreational activities, such as hunting, occur. Hence, the mention of "huntsman".
 
Meanings of difficult words:
 
S.No
Words
Meanings
1
MornMorning
2
Distant Far away in space or time
3
Huntsman A person who hunts
4
Wind To produce sound from an instrument by blowing
5
HornA wind instrument, conical in shape or wound into a spiral, originally made from an animal horn (now typically brass) and played by lip vibration
6
Skylark A bird known for its songs, especially for its high notes
7
Concerta musical performance given in public, typically by several performers
Reference:
National Council of Educational Research and Training (2008). Honeydew. The School Boy-William Blake (pp. 83 - 84). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.