The central theme of the poem "Elementary School Classroom in a Slum" is something like: systemic problems hamper the power of education. The reason this is the theme is that the poem focuses on the idea of education and the barriers that exist to receiving a quality education for students in poverty.
The first three stanzas of the poem focus on the systemic issues that act as barriers to social climbing, showing why education is not an adequate solution to poverty. The first stanza talks about the disease and hunger that students face by describing the way that they look. Students who are "paper-seeming" or "of twisted-bones" are facing systemic issues resulting from poverty that make it difficult for them to learn. Those barriers to learning are then compounded by the effect of colonialism in the second stanza.
The second stanza picks up to show how colonialism has impacted the education of the children. The head of Shakespeare and a donated map are some of the decorations that adorn the classroom. These are symbols of the world that has ensured the children will remain in poverty. They represent class and elitism—the idea that we should educate people in the ways of the wealthy and that this will somehow make them better. But the children know better because they gravitate towards the windows that show the actual reality of their lives instead of the false vision given by the colonizers.
The third stanza of the poem explores the issue of the children watching and envying the upper class. The things they are surrounded by are representative of the things that are not available to them. The final stanza touches on the idea that education could unlock the best for them if they were freed from systemic barriers. The poem describes the concept of overcoming the obstacles by saying:
Break O break open till they break the town
And show the children to green fields, and make their world
Run azure on gold sands, and let their tongues
Run naked into books the white and green leaves open
History theirs whose language is the sun.
If the politicians and others in charge of society work to destroy the barriers that hem the children in, they can create a way out of poverty. The world that education can take them to is described in idyllic language that speaks to the power of education to transform the lives of these children.