Primarily the education budget for school districts. Poorer districts have a harder time meeting some of the newer education standards in effect due to an inability to afford in some cases even adequate educational tools. Richer districts are able to afford much more expensive and effective tools for education.
As an example: Poorer districts are still using projectors for presentations. Richer districts have upgraded to the SMART board, which allows teachers to display not only slides but videos from their computer, as well as allowing free drawing on the virtual board for illustrative purposes.