(Reuters) - Chicago teachers will go on strike against the third-largest U.S. public school district on Oct. 17 if their protracted labor negotiations with the city do not result in a new contract, the teachers’ union said on Wednesday.

A walkout would disrupt classes for about 360,000 Chicago students in kindergarten through high school, following a wave of teacher strikes across the United States over wages and education funding over the past two years, including a week-long strike in Los Angeles in January. African-Americans and Hispanics account for the majority of Chicago’s public school enrollment.

The labor dispute in Chicago has centered on wages and teacher demands for contract language to reduce class size and increase staffing levels for support staff, such as nurses and social workers.

The Chicago Teachers Union announced a “unified strike date” with the labor group that represents Chicago Park District workers and the union representing school security guards, janitors and special education aides.

The union intends to continue to bargain in good faith and that a strike will be the “last resort,” said teachers union president Jesse Sharkey.

“I want no one in Chicago to doubt our resolve. We mean to improve the conditions in our schools,” he said as he was flanked by dozens of school personnel to announce the strike date.

The three unions represent nearly 35,000 public employees in Chicago, according to local media, and a strike at the same time could cause political and policy problems for Chicago Mayor Lori Lightfoot, who was elected in April.

Last week, 94% of some 25,000 dues-paying members of the Chicago Teachers Union voted to give their leaders the discretion to set a date for a strike, which would be the third by Chicago teachers since 2011.

Lightfoot has offered to increase teacher pay by 16% over five years, while the union has countered with a request of 15% over three years.

The union is also insisting on a higher wage floor for school clerks, teaching assistants and others, two-thirds of whose wages, it says, are so low that their children qualify for free and reduced-price school lunches under poverty guidelines.