If all of the illegal immigrants suddenly left Chicago, it would make LOTS of affordable housing available, as well as reduce class sizes and need for more resources in the public schools.
But Chicago (as is common in liberal cities) chooses to provide for the illegals, on the backs of the actual taxpayers. At some point, there will only be a few actual taxpayers left to carry the load.
getting rid of illegals would reduce the demand for teachers. The union sees that as a no-no.