Ah! then at times I drooping sit,
And spend many an anxious hour.
Nor in my book can I take delight,
Nor sit in learning’s bower,
Worn thro’ with the dreary shower.
The third stanza describes how the boy spends his day at school. He claims that he frequently spends a lot of time doing nothing exciting. The phrase "I drooping sit" explains the lack of interest in the boy. Moreover, it is crucial to have a good posture for your health and well being. However, we see that the boy’s back is "drooping", and no one is bothered enough to correct his posture. This also shows the lack of attention given to individuals in schools.

Back to the poem, we see how the speaker claims that he spends several hours anxiously. The speaker is anxious because he is too eager to be out in the open space. He also claims that his books never bring him joy or happiness. Nor can he sit inside his classroom listening to the dull lectures. Here, the phrase "learning's bower" refers to the school. The word bower means a room or a house, and hence, the expression "leaning's bower" can be roughly translated as a place where learning occurs.
Sitting in the classroom makes the speaker weary as he has to go through the "dreary shower" of learning. Here, the phrase "dreary shower" refers to the lectures and education that are dull. The lines reveal the speaker's impatience to free himself from the monotonous school life, get out of the walls of his classroom, and experience the true beauty the summer has to offer. The stanza also reveals the schoolboy's dissatisfaction with the way schoolwork or education is done.
The boy is anxious to get away from the school
Meanings of difficult words:
Droop To bend or hang down heavily
Anxious Worried and nervous; eager to do something
DelightGreat pleasure or satisfaction
BowerA shady and pleasant place (so here, it is a place where sunlight hardly enters)
Thro’ Through
Dreary   Depressingly dull and bleak or repetitive
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.