(CNN) -- Chicago is bracing for a teachers' strike that could affect hundreds of thousands of students next week in the nation's third-largest school district.

Teachers and support staff set a walkout date for Monday, which would mark the first time they have gone on strike in 25 years.

"This is a difficult decision for all of us to make," said Karen Lewis, the union president . "But this is the only way to get the board's attention and show them we are serious about getting a fair contract which will give our students the resources they deserve."

If it happens, it will affect about 400,000 students, including some from neighborhoods struggling with crime and gang problems.

City officials scrambled to minimize the disruption by setting up 144 sites to provide a "safe environment, food and engaging activities" for students if the strike occurs.

"If the leadership of the Chicago Teachers Union chooses to strike, no one will be hurt more than our students, and we are prepared to offer a safety net for families who are not able to access alternative options for their students," said Jean-Claude Brizard, the school system chief executive.

Parents are urged to sign up students in advance to ensure those working at the sites are prepared.

Negotiations with union officials are ongoing with the hopes of averting a strike, Brizard said.

The Chicago Teachers Union said it filed unfair labor practice charges against the Chicago Public Schools this week.

The charges, filed with the state labor relations board, come after changes in working conditions, the union said Thursday.

Changes include new teacher evaluation procedures and the discontinuing of an annual pay increase, it said in a statement.

"Charges come five days before more than 29,000 CTU members are scheduled to walk off the job in demand of a fair labor contract," the union said in a statement. " If contract negotiations have not settled by midnight September 9th, all teachers, counselors, school nurses and other education professionals will go on the strike."

Both sides have not reached an agreement despite 10 months of negotiations.

"Labor talks have been productive on some fronts such as winning provisions for nursing mothers, ensuring textbooks will be available on day one, teachers will have access to functioning computers, and counselors and social workers will have appropriate, private work spaces," Lewis said. "But the bigger issues such as wages, job security and evaluations are on the table and the two sides remain far apart."

A strike will affect nearly 700 schools, according to Chicago Public Schools.