City officals yesterday said they would fire striking teachers and hire replacements if they don't return to work Monday in the most heated of teachers' strikes in seven states that disrupted school for more than 200,000 students.

Other strikes closed classrooms in Illinois, Pennsylvania, Michigan, New Jersey, Rhode Island and Washington.

Officials in St. Louis advertised for new teachers and said they could operate the city's 127 schools with replacements and non-striking instructors. Officials took the names of picketing teachers, planning to turn them over to a court.

Despite the pressure, more than 1,000 teachers attended a boisterous rally in defiance of U.S. District Court Judge Stephen N. Limbaugh, who on Thursday ordered the teachers back to work.

The strike threatened to sabotage a voluntary school desegregation plan acclaimed throughout the country and affected 56,000 students.

In Chicago, a federal appeals court partially upheld a lower court order directing the federal government to provide funding for the city's school desegregation efforts. The Reagan administration had sought to overturn the ruling.

In Newport, Ore., several hundred high school students marched through town demanding the reopening of schools shut down three days into the school year after the district ran out of money and residents refused to vote for more funds.