Chicago Public Schools is taking legal action to try to block the Chicago Teachers Union from holding another strike that the district describes as “illegal.”

The district also wants the union to cover the still-unknown costs of providing more than 250 “contingency sites” for students while schools were closed.

Teachers are holding a one-day strike on Friday to call for more education funding. Union leaders say the one-day action is legal.

But schools CEO Forrest Claypool says state law is clear that the union can’t strike until other steps have been exhausted. The district is filing a complaint with the Illinois Educational Labor Relations Board.

Claypool says it’s important to clearly establish that whether schools are open “is not subject to the whims of the Chicago Teachers Union leadership.”