Both parties agree the 20-year-old formula Illinois uses to fund schools is unfair, but they disagree over how to change it.
Illinois Gov. Bruce Rauner used his veto power to strip millions of dollars for Chicago Public Schools from a school funding overhaul in a move that could mean no districts get state money before classes begin.
The Republican on Tuesday removed help for Chicago Public Schools' pensions along with money the district formerly received in the form of a block grant, along with other changes.
Democratic Senate President John Cullerton had urged Rauner to "do the right thing" and sign the legislation. The bill now returns to the Legislature, where three-fifths of lawmakers in both chambers must either approve or override Rauner's changes. Both options will be difficult.
If neither chamber can muster the votes, the legislation dies. Many districts are concerned about how long schools can stay open without state money.