Teachers in Los Angeles will begin a strike on Monday after failing to reach an agreement with the school district in an ongoing contract negotiation.

“We are at an impasse,” union president Alex Caputo-Pearl said Friday, adding, “get ready, because, on Monday, we will go on strike.” The industrial action will affect more than 600,000 students in the nation’s second-largest school district, which has hired hundreds of substitutes to replace the 34,000 teachers expected to walk out on Monday morning. The staffing level is expected to be just 8% of a normal day.

The strike was originally scheduled to begin last Thursday, but the union postponed it “in order for clarity and to allow members, parents, and our communities to plan.” The union also encouraged parents to join them on the picket line.

Some parents intend to keep their children out of school, but that won’t be an option for sympathetic parents who don’t have alternative childcare arrangements. The district said students are expected to be in class, regardless of the action — the first teacher strike in California for 30 years.

United Teachers Los Angeles is asking for a 6.5% raise that would take effect immediately and apply retroactively through fiscal year 2017. They’ve also demanded smaller classes and more support staff, arguing that the district has enough money to fund these demands. The district, however, says it would go bankrupt if it were to meet all of the union’s demands.

In a last-ditch effort to reach an agreement with the teachers, the district on Friday offered a proposal that included hiring 1,200 additional teachers and reducing class sizes — for fourth to sixth grades, the class size would go from 36 to 35; for high school, classes would be capped at 39, down from 42. The union called the proposal “woefully inadequate”.