UPDATED 08/10/12 6:59 a.m.

CHICAGO (CBS) — Thousands of Chicago kids head back to school on Monday, and there is a troubling message for parents.

Despite the resolution reached between the Chicago Public Schools and the Chicago Teachers Union on the longer school day, the contract as a whole remains unresolved. And the union is warning teachers may head to the picket lines instead of in the classroom once all the students return in September.

LISTEN: WBBM Newsradio’s Bernie Tafoya reports

CTU president Karen Lewis said when about a third of CPS students who are on the Track E schedule return to school on Monday, there is no way the contract will be resolved. The contract expired on June 30.

Lewis said negotiations have been slow-going.

The union sent out a news release. Under the headline, “Update on contract negotiations,” the release reads, “Contract is not settled,” and, “Members continue to prepare for a work stoppage.”

The release goes on to say if there is a strike, it would happen in September when most of the teachers return to the classroom.

The CTU says even though both sides have reached an agreement on the longer school day, issues such as job security, health benefits and teacher pay still remain unresolved.

Speaking on the CBS 2 Morning News Friday, CPS chief executive officer Jean-Claude Brizard said he was “surprised” by the union’s remarks.

“The fact is that we met yesterday – marathon session. We got some things accomplished,” Brizard said. “Things are moving. We’re not done. Clearly we have a lot to cover. But things are moving, and they’re moving in the right direction.”

When asked what his contingency plan would be if a strike went ahead, Brizard said: “You know, 500,000 students, 26,000 teachers, there’s no way in the world that we can run schools without our teachers. We need them to be there.”

But Brizard expressed confidence that teachers want to be working and are excited about the new school year. He also said school will start as scheduled for sure in the Track E year-round schools on Monday.

“This week, in fact, I was at three schools this week with teachers who are preparing for Monday, and let me tell you – they are excited. They are working Monday with a full school day. Monday, we did technology in different classrooms,” Brizard said. “We have people who are ready to come back and work. Schools will open on time on Monday – 120,000 kids, 240 schools.”

Other sources at CPS say the union news release is just a tactic by the union to try to drum up support.

The union voted back in June to authorize a strike, and both CPS and the union last month rejected a fact-finder’s report that was supposed to resolve salary issues that have been a sticking point.

Under the law, the union must give the district a 10-day notice of its intent to strike.

School for most CPS students starts on Sept. 4, the day after Labor Day.