Back-to-school week began more peacefully around the country yesterday than it has in several years, with fewer teachers on strike and more students in classes.

Summer vacation had unscheduled extensions in only five states, with nearly 9,000 teachers on the picket lines and an estimated 140,000 students affected.

But educators' attention was focused on Detroit, where a possible strike could more than double the numbers. The Detroit Federation of Teachers voted yesterday to extend its contract until Friday while talks continue. Officials were not optimistic, and a strike would involve 11,000 teachers and 211,500 students.

Union officials said the listless economy is responsible for the relative quiet: about 50,000 teachers have been laid off and the rest were less inclined to go to the mat over pay raises from hard-pressed school districts. Where they did, the issues were economic: pay hikes, job security and classroom hours.

Still, Michigan was the state hardest hit by discord, just as it is one of the hardest hit by the economic slump.

Detroit school board members, trying to deal with a $25 million budget cut, asked teachers to swallow a 9 percent pay cut. The teachers, whose average salary is $22,500, want their current contract extended. "If we don't work anything out . . . come Monday we'll be on strike," federation president John Elliott told the Associated Press.

Teachers walked out in seven other Michigan districts, joining those in four districts already out and bringing the total to 4,000 teachers on the picket lines in the state. An estimated 50,000 students were affected in Kalamazoo, Southfield, Troy, Traverse City, Ferndale, Suttons Bay, Lake Orion, Lake City, Novi and Fenton.

In Pennsylvania, nine school districts suffered strikes involving about 1,000 teachers and 33,000 students, but Philadelphians went back to school with a new contract giving teachers 18 percent more over three years. It was the first time in three years that the city has not had a walkout.

About 1,100 teachers have been home since Aug. 31 in East St. Louis, Ill., rejecting a board offer of a 5 percent raise of $3,200 over two years and keeping 20,000 students on vacation although the schools stayed open. No negotiations were scheduled.

Another 2,600 teachers are striking in five other Illinois districts serving 29,000 students.

There are 273 teachers on strike in North Olmstead, Ohio, but 5,000 teachers are working without a contract in Cleveland while talks continue. A walkout there would affect 68,000 students.

The 2,600 students in North Plainfield, N.J., got an extra day off yesterday while school officials negotiated past a strike deadline from the town's 203 teachers.

As in Cleveland and Detroit, talks were continuing while teachers worked in Chicago, where a vote on a proposed contract is set for Sept. 16. Classes are expected to begin as scheduled today for 450,000 students.

Some districts reported happy settlements. The 8,000 teachers in Dade County, Fla., which includes Miami, got wage increases of 10 to 20 percent, according to the American Federation of Teachers.

AFT spokesman Scott Widmeyer said the 716 high school teachers in the northwest Chicago suburb of Schaumburg, Ill., got "what may well be the best contract settlement in the nation this year," a 39 percent raise over three years. Personnel director Donald J. Skinner, however, said the number was deceptive.

"Under our previous three-year agreement, our teachers actually had fallen behind," he said.