PITTSBURGH (AP) -- Teachers in three Pennsylvania school districts went on strike Tuesday, while classes began as scheduled for many more of the state's roughly 1.7 million public school students. In northeastern Pennsylvania, members of the Old Forge Education Association in Lackawanna County began walking a picket line after negotiations broke down last week. No new talks were held over the weekend. "When your hand is forced, this is where you end up," said Shawn Nee, the union's vice president, said Tuesday. In nearby Luzerne County, Wyoming Area School District teachers went on strike Tuesday after three years of fruitless bargaining. Union president Melissa Dolman said that if there is no progress, teachers will not return to the classroom until Sept. 30. In western Pennsylvania, union representations and officials in the Shaler Area School District met for about six hours Monday with a state-appointed mediator but were unable to reach agreement. Melissa Ravas, president of the Shaler Area Education Association, said teachers were expecting a new offer Monday but didn't receive one. Shaler schools Superintendent Wesley Shipley said the next negotiating session is planned for Wednesday evening. Officials said that under state law, all instructional days lost due to the walkouts must be made up by the end of June of next year. Tim Eller, a spokesman for the state Department of Education, said that if a strike reaches a point where there aren't enough days left in the school year for students to complete studies, the Secretary of Education can petition a local court of common pleas to end it. That day will be reached on Sept. 24 for Shaler, Eller said, but calculations aren't complete yet for the other two school districts. Elsewhere, students arrived for their first day of school in many of the state's 500 school districts, though some began classes before Labor Day.