At issue is how much money schools spend per student. In a funding system largely fueled by local property taxes, New Trier Township spent nearly $17,000 per student in 2005-06 and Sunset Ridge spent about $16,000, while Chicago Public Schools spent an estimated $10,400 per student.
The leaders of the protest seem to want to change the system so that all schools in the state get the same amount of per-pupil funding. This would be better than the current system, but imagine all the bureaucracy needed to work this out.
A much simpler, more elegant solution is to just attach the funding to the student and let the student and their parents decide where to go to school. This is how they do it in Sweden.