Updated at 8:20 p.m.

The Boston Teachers Union announced Thursday that they are suing to block Boston Public Schools from reopening schools in-person and requiring educators to return to campuses, after hours of negotiations with Mayor Marty Walsh and BPS.

Union officials filed a temporary restraining order against the city and BPS Thursday afternoon, citing a risk of “irreparable harm” done to both students and teachers if in-person classes are held in the midst of an uptick in the city’s COVID-19 infection rate.

“Severe illness and death are the risks of this decision, which cannot be compensated by payment or money damages,” BTU President Jessica Tang said in a news conference Thursday. “To be clear, we want to ensure in-person services for our highest need students, but we need a plan for it.”

After the citywide COVID-19 positivity rate rose to 4.1% on Wednesday, Walsh announced that phased reopening of schools would be pushed back from the scheduled Oct. 15 date to Oct. 22. Some teachers would still be required to come to school campuses to teach “highest-need” students, Walsh said, including students with disabilities and those experiencing homelessness.

“Educators and staff who provide education to such students will be expected to report to their school buildings on Thursday,” BPS Superintendent Brenda Cassellius wrote in an email to BPS staff on Wednesday.

Tang said this requirement is a direct violation of an agreement that BPS made with the BTU last month which explicitly prohibits schools from “directing” teachers to come in if the citywide COVID-19 rate rises above 4%.

“The superintendent's disingenuous interpretation of the agreement is simply wrong,” Tang said. “We feel that the superintendent's threats of discipline constitute an effort to intimidate educators — and that those efforts are at odds with our agreement.”

In an emailed statement Thursday evening, a spokesperson for BPS and the city of Boston did not directly respond to Tang's statements, but said that BPS based their decision to require some teachers to return in-person on guidance given by public health experts at the Boston Public Health Commission, the state’s COVID-19 Task Force and the Deptartment of Secondary Education.

The spokesperson quoted Cassellius as saying that the school district's top priority "is the health, safety and well-being of our students, staff and families."

"While the majority of BPS students are currently learning remotely, we are proud to provide in-person learning opportunities for our students with the highest needs, with the supports and services that they require and deserve," Cassellius said in the statement. "BPS deeply appreciates the hard work, dedication, and public service of all our educators and staff."

The spokesperson also said that Walsh "wholeheartedly believes that special consideration must be given to our highest needs students who rely on the in-person instruction and support offered by their teachers in a classroom setting, and that we cannot take this away from them when there’s an opportunity, backed by public health, to have them in schools."

Emails acquired by GBH News on Wednesday show that principals and other top officials from some BPS schools appear to be attempting to pressure teachers to return to in-person schooling.

“Failure to report may constitute insubordination and lead to disciplinary action,” one email from officials at Excel High School in South Boston reads.

The exact same language appears in an email sent to staff from the principal of the Richard J. Murphy school in Dorchester, followed by a plea: “We need ‘all hands on deck’ to serve all our students during this challenging time.”

Officials at the Thomas A. Edison K-8 school in Brighton pointed out that working in-person “has become optional, per the safety-focused agreement put in place with BPS,” but still urged teachers to return to campus. “Out of great concern for our highest needs and special education students,” the email to staff reads, “we know many educators will continue to work from buildings.”

The same phrase regarding “disciplinary action” appears in several other emails from BPS school leaders, described by the sender as a “required message” that was sent along with the superintendent’s email.

“Things are a bit tense between BPS and BTU,” a school official from the Boston International School wrote to staff in an email Wednesday. “We all chose a hard job. We could not have imagined it would get this hard.”

Tang described the language sent to BPS educators as “quite threatening,” and said the directive included a “range of disciplinary action” including the firing of teachers.

“We think that was absolutely the wrong approach, when teachers have been working so hard to ensure the safety of our students,” Tang said. “And because they are concerned about the conditions of our buildings and the number of staff going in that don't need to go in, as well as the safety of our students, they are being penalized and intimidated.”

Tang also called on BPS to provide evidence that school buildings have been equipped with sufficient air ventilation and air quality. “If buildings are not safe for educators, we don't think they are safe for students either,” Tang wrote in a statement Thursday, “particularly for our highest needs students.”

Tang said she understands that certain students require in-person teaching, but that this current proposal “doesn’t make sense.”

“This is resulting in schools with just a handful of students, but dozens of adults, unnecessarily in the buildings — risking the chance of spreading the virus to students and adults alike,” Tang wrote. “In-person students should have teachers who can focus on their in-person services while others are focused on remote students. We will keep fighting for what is best for our students, families and educators both for their academic needs and also for their health and safety.”

Correction: An earlier version of this article incorrectly referenced an email from officials at Excel Academy Charter High School in East Boston. The email was in fact from officials at Excel High School in South Boston.