Teachers walked picket lines with little progress in negotiations in suburban New Orleans and Oklahoma City while thousands of others were on strike in five other states.
While some schools have not begun classes, at least 210,000 students would be affected if the job actions continued through next week.
Some districts are already employing temporary help to operate schools, whose unionized teachers are staying off the job in disputes involving pay, vacations and grievance procedures.
About 2,300 teachers remained off the job in seven Michigan school districts, and Keith Geiger, president of the 90,000-member Michigan Education Association, predicted the walkouts will spread to "20 to 40" other districts, including Detroit public schools.
School officials in Oklahoma City called an afternoon news conference to discuss the strike by more than 1,000 teachers, affecting 43,000 students. But they refused to send negotiators back to the bargaining table, saying it may be illegal to do so.
In Louisiana, union leaders said a strike by more than 2,000 teachers affecting 64,500 students in Jefferson Parish seemed to be gaining support.
School officials said there were 50 fewer teachers reporting for work yesterday than on the first day of the walkout Friday.
Other strikes were reported in some school districts in Ohio, Illinois, Pennsylvania and Indiana. A strike in Missouri's Park Hill district was settled Sunday.