Both sides had met into the early-morning hours Sunday after the contract expired at midnight, with the union reporting signs of progress.
"Hopefully, they're building on that progress," union spokesman George Jackson said Sunday.
Fernando Gallard, the district's spokesman, said he could not confirm whether the talks had been fruitful.
"All I can say is we'll continue to negotiate," he said. "We're hopeful that we can resolve it as quickly as possible."
Teachers will work under the terms of the expired contract while negotiations continue. Classes for 136,000 schoolchildren begin in a week.
The district seeks $103 million in concessions from the union, including wage cuts ranging from 5 percent to 13 percent and changes in health-care coverage, plus flexibility to assign teachers to schools without regard to seniority and to eventually award raises based on performance.
The union has said it would forgo a year's raises and agree to changes in health coverage that could save the district millions but would not accept wage cuts.
Union leaders have said they won't seek a strike authorization at Monday's membership meeting. Under state law, teachers could lose certification if they strike.
Contact Ben Finley at 610-313-8118, firstname.lastname@example.org, or follow on Twitter @Ben_Finley.