The Los Angeles Unified School District, now in the midst of the nation’s largest public works program, extended its project labor agreement on Tuesday, favoring union contractors.

The district’s Board of Education voted to extend the project labor agreement first negotiated 10 years ago for another five years, through 2013. Under the agreement, which is one of the largest ever implemented, all contractors must hire at least a portion of their workforce from union-run hiring halls. In return, the building trades unions promise not to strike or otherwise hold up work if disputes with contractors arise.

Also as part of the project labor agreement, 50 percent of the construction workers hired must reside within the district.

The school district has already completed 74 out of the 132 planned new schools in its $20.3 billion construction and modernization program, as well as more than 17,000 smaller-scale upgrades to existing school facilities.

“Between now and 2012, we will be completing more than one school a month. This would not be possible without the partnership with the unions,” district facilities chief Guy Mehula said in a statement.