Theory:

How shall the summer arise in joy,
Or the summer fruits appear?
Explanation:
 
The poem’s final stanza, which is made up of two lines, poses a question to the speaker’s parents. Resuming the thought from the previous stanza, the speaker asks his parents how can summer be enjoyable when there is a lot of destruction happening within him. His freedom and joy are clipped and thrown away, and yet, he is expected to be happy and content.

He wonders how a plant can produce fruit when the bud and blossoms are plucked away. Likewise, how can a child grow into a talented, caring, creative, and successful person if the very childhood is stolen away from them?

The poem ends in a rhetorical question posed not only to the parents but also to society. The poem is a call to action against the constraining, cage-like educational system and the intolerable pressure and expectations forced upon the kids by their parents.
 
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When schools become prisons
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.