Elgin Area School District U-46 union leaders filed a 10-day intent to strike notice Wednesday, the day after a 15-hour negotiation session failed to produce a contract for teachers.
State law requires that education officials meet with a mediator and give 10 days' notice before striking.
Sessions with a federal mediator are scheduled for Dec. 6 and 11.
Elgin Teachers Association President Tim Davis emphasized the move was routine and not a sign that teachers were ready to walk the picket line.
"At this point it's purely procedural," Davis said Thursday. "A strike is a last resort. I don't think we're there."
The union also filed an intent to strike notice during negotiations for the 2004-07 contract, but a deal was reached before the start of school.
Union and district officials opted to call in a mediator after Tuesday's marathon bargaining session. Officials had told union members they would engage in mediation if a deal was not reached by Nov. 19, but they then extended the deadline to Tuesday.
"We were not at an impasse. I thought we had the potential to settle Tuesday," Davis said, "but we had to be true to what we stated to members."
District officials agreed the sides have made headway after meeting three times since members rejected a tentative three-year deal in October.
"The board's team believes it is very close to a settlement," said a memo sent Wednesday by the district's bargaining team to administrators.
Class sizes and case loads are the sticking points, Davis said, though any fix to those issues could affect teacher raises.
"Everything on the table will require a financial commitment," Davis said. "There are no easy solutions, even with all of the brainstorming we've done."
The 2,400 members of the state's second-largest teachers union have been working under the terms of an expired contract since the beginning of the school year.
The deal that members opposed, by a vote of 1,183 to 1,125, called for average raises of 6.1 percent in the first year of the contract and between 4.4 percent and 5.7 percent, determined by the rate of inflation, in the second and third years of the contract.
In a recent survey, teachers who rejected the deal listed the raises in the second and third years of the contract as the second greatest factor influencing their vote.
Class sizes and case loads topped the list.
The deal would have provided two teacher aides at each high school and one aide at each middle school to relieve excessive class sizes, a fix that teachers called inadequate.
Davis said the district needs to find immediate, short-term and long-term solutions to large class sizes and case loads.
According to union statistics, there still are about 160 fewer union members, including teachers and support staff, than in 2004, when the district faced a $40 million operating deficit and instituted drastic budget cuts and layoffs.
"The district had to get its financial side in order," Davis said. "But I think with such an intense focus, they took their eyes off the academic program."
U-46 teachers went on strike seven times between 1978 and 1991. The last work stoppage lasted 21 days.
"The board's team has spent countless hours during the first round of bargaining which resulted in the original tentative agreement, and remains dedicated to reaching another tentative agreement," the U-46 memo to administrators said.