A tentative deal has been struck between the Philadelphia School District and its blue-collar workers' union, officials said late Wednesday.

The deal - which must be ratified - appears to avert layoffs and the privatization of 2,700 jobs of members of SEIU 32BJ Local 1201: bus aides, mechanics, cleaners, building engineers, and other workers.

Union officials confirmed the deal, but said they would release no specifics - including the number of layoffs averted - until members saw the terms.

The potential agreement comes after months of negotiations and protests over the possible layoffs. About 500 employees were to have lost their jobs Sunday, with the rest laid off by the end of the year.

Union leaders said they had put at least $20 million in concessions on the table.

"Under the four-year agreement that saves thousands of jobs, blue-collar workers will make direct contributions to keep schools open and to help the district solve budgetary problems," union officials said in a statement.

Many of the union's members are paid less than $30,000 a year, some less than $20,000. Givebacks could mean $6,000 less in workers' pockets annually, union leaders said last month.

Members will vote on the contract July 20. If it is ratified, the SRC would have to approve it.

"This agreement will help keep public schools open, provide a clean and safe learning environment for our kids, and save jobs for thousands of hardworking men and women," union Chief George Ricchezza said in the statement.

The cash-strapped School District - its deficit for 2012-13 could be as much as $282 million - has said it needed at least $50 million in savings from "modernizing" transportation, custodial, and maintenance services. It had solicited bids to see how much private companies would charge for the work.

But some City Council members made it clear that they wanted the district to come to an agreement with 32BJ, and part of the $40 million in new city money Council had agreed to give the district comes with strings attached.

Union leaders had also said they were worried by the news, first reported in The Inquirer, that the School Reform Commission was lobbying in Harrisburg for legislation that would give it the absolute right to cancel union contracts and impose terms on workers.

Those efforts stopped when members of the Philadelphia Democratic House delegation, angry that they were kept in the dark, confronted the SRC.