The executive budget Mayor Bloomberg will unveil tomorrow won’t call for any reduction in the size of the city’s teaching corps, according to sources at the City Council.

In each of the last two years, the city has narrowly avoided teacher layoffs but has still seen the number of teaching positions drop because of attrition, last year by 1,800 spots. For next year, a hotly debated line in the mayor’s preliminary budget called for the city to leave about many teaching spots unfilled. The city pegged the reduction at about 1,100 positions, but City Council members said the real shortfall would have cost 2,500 jobs.

Council members continued to be distressed about the proposal even after Chancellor Dennis Walcott assured them during a hearing in March that the final budget would find funds to close the gap and make attrition unnecessary.

“It’s my goal and our hope to make sure that the budget stay flat without having any cuts to our schools,” Walcott said at the time. “We’re going to work very hard within the system that any type of absorptions be done centrally.”

At a press conference this morning, Council Speaker Christine Quinn said concerns about attrition had persisted.

“In the council’s budget response, we articulated very significant concern about the level of proposed attrition within the department of education,” Quinn said. “I hope we get action in that area.”

The council sources could not say whether deep cuts slated for after-school programs would be rolled back. Advocates have been pressing the city to restore some of the proposed after-school cuts, including with a barrage of phone calls this week, and are planning a rally on the steps of City Hall for moments after the executive budget presentation on Thursday.

Next year would be the first without planned attrition since the Department of Education imposed sweeping hiring restrictions in 2009, and it suggests that conditions are right for some of those restrictions to be lifted as schools enter the thick of hiring season.

United Federation of Teachers President Michael Mulgrew said the budget news should augur new possibilities in class size reduction, a union priority.

“This is good news, Mulgrew said in a statement. “We’ve lost thousands of teachers over the last three years to attrition, and class size is higher than it has been for decades. As the city stabilizes its teacher losses — and ideally starts hiring more — we should be able to start bringing those class sizes down.”

But whether the city can continue to rebound is an open question. According to WNYC, Bloomberg is set to say that the city’s revenue is not growing as quickly as city analysts had predicted.

In the next two months, the mayor and council will joust over the budget’s details, and a final budget must be set before July 1.