Members of the Valley View School District 365U teacher’s union voted to authorize a strike after their previous contract expired and talks stalled with the district.

Local 604 of the American Federation of Teachers which represents the district’s teachers, paraprofessionals, maintenance crews, nurses, security, and other staff members, took the vote on Aug. 27-28, and approved the possibility of a strike by an overwhelming majority.

Nearly 96% of the membership voted to authorize a strike if negotiations do not improve.

Authorizing the strike does not guarantee that the teachers will walk out of the classroom, however, it leaves the option open to the union’s bargaining team.

One of the largest sticking points for the negotiations revolves around a district proposal to extend the school day at the district’s two high schools, Bolingbrook High School and Romeoville High School, by 20 minutes.

The plan was originally proposed for the entire district, but was later changed to only impact the high schools during negotiations.

The plan could potentially require staff and students to begin their school days at 7:10 a.m., but the district has not presented a detailed plan for the modified schedule, which has frustrated teachers, parents, and students.

“The Board is trying to force an incomplete plan that no one asked for, with no stakeholder input, and no consideration of how this will affect bus schedules, district costs, logistics, or staff workload,” Valley View Council President Jared Ploger told NBC News. “On top of that, studies show that earlier start times can hinder, rather than help, student learning. It just doesn’t make sense.”

Parents marched in a solidarity picket for the teachers outside the district office in Romeoville on Friday, in an event that was planned before the results of the strike vote were officially announced.

“As parents, do we want the strike to happen? Of course not, but we want our teachers and staff to get an unambiguous and fair contract,” said Ruth Michalsen, the parent who organized the picket.

Valley View District 365U’s administration did not return a request for comment about the contract negotiations or the modified schedule.

According to the district website, the proposed change would move the school to an eight-period day, though it is unclear how those periods would be divided.

It is also not stated if the extra time would be added to the end of the school day or if it would make the day start sooner, either eliminating zero hour or pushing it’s start back even earlier.

The union’s contract with the 10th largest school district in Illinois expired on Aug. 15, one day after students returned to school for the fall semester. If negotiations do not reach a conclusion that satisfies both parties, the union can call a strike as soon as Sept. 15.