It has become a familiar pattern in the District. The city and Washington Teachers’ Union near an agreement on how schools should reopen. Both sides indicate that a finalized deal could be imminent. And then, at the 11th hour, it falls apart.

Last week, union president Elizabeth Davis circulated a “tentative agreement” to members. On Tuesday, she decided not to sign it, continuing to disagree with the school system over whether in-person teaching should be optional for all teachers.

Davis also wants to understand what coronavirus metrics would trigger schools to close, according to union spokesman Joe Weedon. The school system does not need a deal to reopen schools, but a signed agreement would make it more likely that teachers would willingly return to classrooms. The city already abandoned plans to resume in-person teaching this month because it didn’t have sufficient staffing.

“I can’t say that I’m surprised,” Mayor Muriel E. Bowser (D) said at a news conference Wednesday. “She’s backed away from other tentative agreements as well. And it should be apparent that the goal posts, unfortunately, continue to move.”

The collapse of the deal arrives as coronavirus case rates are rising in the city.

Schools Chancellor Lewis D. Ferebee and Davis have been negotiating since the summer, and this is the first time the union has publicly said an agreement is contingent on more specific guidance on when schools should be allowed to reopen. Previous drafts of the agreement called on the city to follow local and federal health guidelines to determine when schools can operate.

But union representatives say they want more clarity on when local health guidelines would deem it unsafe for schools to be open. The District is in Phase 2 of reopening, which recommends that schools be allowed to reopen with social distancing and other safeguards.

With rising case numbers and infection rates, the city could revert to some aspects of Phase 1. In Phase 1, according to health guidance, schools should not operate in-person. Bowser has so far decided to keep the city in Phase 2 but said Wednesday that she would dial back some of the Phase 2 openings. She has not indicated whether this would affect school reopenings.

“With cases of COVID rising across the region and without clear guidance from the Mayor as to what level of spread would cause our schools to close, we cannot move forward with an agreement that could force anyone, regardless of preexisting conditions or their living situations, back into an in-person classroom,” Davis wrote in a statement.

Davis continues to demand that in-person teaching be optional, even if a teacher is not considered high-risk.

The tentative agreement had said returning to physical classrooms would be optional for all teachers in the second quarter of the academic year, which began this week and runs through January.

After that, teachers who do not have exemptions would need to return if student demand for in-person learning exceeds the number of teachers who volunteered to come back. The agreement also said the union could renegotiate terms ahead of the third and fourth academic quarters, meaning nothing was permanent.

Davis said in an interview last week that she felt “okay” with these terms, but on Wednesday, she said she wanted more protections for staff members.

The school system did not respond to a request for comment.

D.C. Public Schools reopened 48 classrooms at 25 schools Wednesday under a program it calls CARE. In CARE classrooms, students continue their virtual learning at school under the supervision of nonteaching staff. Davis said she still has questions about building safety.

The union is now hoping the D.C. Council passes emergency legislation that would more strictly define how schools can reopen. The original legislation, introduced by council member Elissa Silverman (I-At Large), says the school system would be prohibited from opening more than two classrooms per building if the city is hitting any Phase 1 health metrics.

Silverman said she spoke with school system staff members before writing the legislation last week and was under the impression that schools would not initially open with more than two CARE classrooms. However, many schools did open with more than two classrooms, so Silverman said she is rewriting parts of the legislation so it doesn’t interfere with students already in school.

The original legislation would also require the mayor to give the D.C. Council 21 days’ notice of any reopening plan before she announces it. Silverman said she is open to changing that time frame.

Ferebee and Bowser have been charged with not soliciting enough public feedback about their reopening plans, and Silverman hopes this would better involve the public in the process.

“I felt there was a need to be a vehicle for transparency and collaboration in the school building reopening process,” Silverman said. “You need to have confidence from all parties to reopen schools.”

It’s unclear if Silverman will have the nine votes — a supermajority of the 13-member council — to pass this legislation. Ferebee and D.C. Council Chairman Phil Mendelson (D) have both said they are against it.

Julie Zauzmer contributed to this report.