After 21 months of negotiations over a new teachers contract, the Boston Teachers Union and the School Department declared Tuesday night that they had reached an impasse and will ask the state to appoint a mediator in hopes of resolving a stalemate over pay and other issues.
The two sides said they will file the request together Wednesday morning with the state Division of Labor Relations. The mediator would meet with both sides jointly and independently to help them reach an agreement.
The move came amid growing pressure from city and business leaders, parents, students, and education advocates to bring negotiations to an end and settle a contract that could usher in a number of new initiatives to overhaul the city’s schools, such as adding 30 minutes a day to each of the 125 schools.
“Negotiations have gone on long enough,” said Richard Stutman, the teachers union president.
The union was the first to declare the impasse, around 5:45 p.m. during a negotiation session that had started at 2 p.m. and was scheduled to end at 8 p.m. It was one of several sessions that have taken place this month, and session had been scheduled for Friday.
Shortly afterward, the union issued a press release announcing the impasse and its intent to seek mediation. It also said it invited the School Department to jointly make the mediation request—an offer school officials later accepted.
“Generally speaking, we agree we need to move this forward,” said Matthew Wilder, a School Department spokesman.
The mediator would only have limited power. That person can only make recommendations, but the voice of an impartial third party could carry considerable sway among two parties that have been unable to see eye to eye on many issues
Among the key unresolved issues are annual pay raises for teachers, a new way to evaluate teachers, and whether they should be paid for teaching a proposed additional 30 minutes a day.
The union has accused school officials of refusing to negotiate on several issues involving teacher pay and compensation, while the School Department says it is trying to make prudent use of public tax dollars.
Stutman blamed much of the inability to reach an agreement on Superintendent Carol R. Johnson, who he said has attended only one negotiating session. Her predecessors, he said, routinely took part in the negotiations.
“The superintendent should be at the table,” he said. “She is the chief decision maker. ... You can’t make a decision without a decision maker.”
Wilder said the School Department’s negotiators have the authority to make decisions at the bargaining table. He also defended the superintendent’s role in the negotiations.
“The superintendent is managing a 125-school district, and there is a lot on her plate,” Wilder said. “She is well aware of what is happening every day in the negotiations.”
The impasse was announced hours after Mayor Thomas M. Menino used a major policy speech to fire a fresh salvo at the teachers union, saying that stalled contract negotiations will prevent the city from putting the right teachers in the right classrooms.
“The union refuses to accept a $32 million raise for teachers,” Menino said in his annual address before the Boston Municipal Research Bureau, an 80-year-old fiscal watchdog funded by businesses and nonprofits.
“Unfortunately, we must start planning for next year with the old process in place. A process that puts teachers with mismatched skills in mismatched classrooms. That’s wrong,” he said.
Responding to the mayor’s comments, Stutman said the city’s contract offer “looks like a lot, but it really isn’t.”
The $32 million in raises would be spread over three years and would amount to an average pay increase of about 1 percent for the city’s 7,000 teachers, paraprofessionals, and substitutes. The union has asked for what Stutman said would be an average raise of roughly 3 percent.
“We share [the mayor’s] frustration,” Stutman said. “We are very frustrated that the school superintendent has barely set foot in  bargaining sessions. I think if the mayor looked a little deeper, he’d find a nonchalance in his own school” administration.
The teachers have been without a contract since Aug. 31, 2010, when their old agreement expired, making it one of the longest periods that teachers have worked without a contract.
Speaking in a ballroom at the Seaport Boston Hotel, the mayor also touched on what he described as a successful partnership between government and business that has created a development boom in the city, making Boston an economic model for the nation.
“Cranes are in the air and businesses are opening on the ground,” Menino told the crowd, which included hundreds of business and civic leaders. “There is no doubt that Boston is a rising sun.”
Martin Finucane of the Globe staff contributed to this report. Vaznis can be reached at Jvaznis@globe.com. Ryan can be reached at ARyan@globe.com