BURLINGTON, Vt. (AP) — About 400 teachers in Vermont's largest city went on strike Thursday, two weeks after the school district imposed contract terms and a day after a last-minute bargaining session with the help of a mediator failed to reach an agreement.

Teachers picketed outside Burlington's six elementary schools, two middle schools and the high school, while about 4,000 students had an unexpected day off early in the school year.

Burlington Education Association President Fran Brock said the two sides had been making progress on economic issues they disagreed on, such as a pay increase. But she disputed an assertion by the school board that the two sides had reached an agreement and then made additional demands at the bargaining table.

Brock said the main sticking point in the negotiations is scheduling time for teachers to prepare outside the classroom and to work one-on-one with students. She said an agreement had been reached for teachers at the high school, but not at the elementary schools.

"We're the experts," Brock said at a Thursday morning news conference outside the high school where teachers were protesting. "We know what we need to do; we need the time to do it so that the children are not cheated out of the education they deserve. Basically we need to figure out effective ways to use the time in the school day."

In a recent statement, the school board said the two sides had "resolved operational issues" at the high school. The board offered teachers an 8 percent pay increase, a three-year contract and additional health insurance savings, but the union rejected them and chose instead to strike, according to the statement.

Brock denied the board had offered the raise. "Do you really think we would turn that down?" she said.

There are no immediate plans for more negotiations.

The Burlington School district canceled classes and other school activities for Friday. It said on its website that Superintendent Yaw Obeng is working with administrators to determine if some high school homecoming athletic events can be played as scheduled. Non-teacher coaches and volunteers would be needed, the district said.

The district has about 4,000 students in six elementary schools, two middle schools and one high school.

The school board voted Sept. 1 to impose contract terms on the teachers. In neighboring South Burlington teachers are also working under an imposed contract.

It's the first strike in the Burlington school district since the late 1970s.

On the picket lines, teachers referred all questions to union leaders.

"It's terribly inconvenient," said Shoshannah Boray, the parent of two children in the Burlington school district who came out to support the teachers. "I'm very, very worried for our children, but the teachers deserve respect and they deserve to be treated respectfully."