He compares himself with a bird in a cage or a plant that withers because he is not happy with his school life and he is unable to enjoy his freedom to play during school time.
The poet wants to convey a very practical lesson through this poem. He wants that we should not bury the childhood of a child under the burden of heavy bags and books as a child loves to be closer to nature, stars, flowers than to this materialistic world.
This school boy loves to spend summer mornings and hearing singing of bird, song of skylark.
It means the dull/uninspiring life at school with lots of work and no play.
No, the school boy is not a happy child, because he did not like to study in the class.