Even if no settlement is reached, the teachers must return to work on Sept. 24, state education officials have told the union and the district.

The two sides are far apart; the main issues in the 5,900-student district are wages and health-care benefits.

The 512-member union's latest proposal before the strike began on the first day of school was for a 5.98 percent wage increase in the first year of a four-year agreement, 9.4 percent in the second year, 7.14 percent in the third and 6.9 percent in the fourth. The school board proposed a three-year contract with increases of 2.5 percent each year.

The school board is also seeking to eliminate the teachers' highest-tier health plan and wants higher premium contributions and more co-pays for the remaining two. The union wants to leave all three plans in place with the same percentages of premium contributions that are now in effect, and wants improvements.

Under Pennsylvania law, the teachers must go back in time for students to receive 180 days of instruction by June 15 of next year. The Sept. 24 date would allow that, counting in state-set holidays such as Thanksgiving and Christmas and including five district-designated holidays.

If the strike lasts until Sept. 24, the two sides would enter a state-mandated final best-offer arbitration process that could take many months. A three-member panel would issue a recommendation after holding hearings. If either of the two sides rejects the panel's proposed settlement, the teachers could strike again, but would have to return to work in time for students to receive 180 days of classes by June 30.

The Souderton strike is the only one in the Philadelphia suburbs this fall, but on Wednesday, teachers in Delaware County's Springfield School District authorized their leadership to call a strike if necessary. No strike deadline has been set; bargaining was scheduled for last night.