Teachers in Chicago and Seattle went on strike yesterday, one day before the scheduled start of classes, while teachers in Philadelphia shouted their overwhelming approval of a three-year contract, averting a walkout.

Strikes by teachers in 10 other school districts in Pennsylvania, Illinois and Michigan disrupted the resumption of classes for more than 76,000 students.

The Chicago Teachers Union, representing 28,000 teachers in the nation's third-largest school district, launched its third walkout in as many years after rejecting the 430,000-student district's offer of a 3.5 percent salary increase. The union wants a 9 percent raise.

Aides to Illinois Gov. James R. Thompson (R) met yesterday with both sides to present a proposal, the terms of which were not disclosed. A coalition of 17 unions representing 12,000 other Chicago school workers, including painters and engineers, also struck yesterday.

The 19,000-member Philadelphia Federation of Teachers ratified by voice vote a $250 million, three-year contract that includes pay raises totaling 10 percent. The teachers in the 197,000-student district now earn an average of $30,000 a year.

In Seattle, teachers voted 1,684 to 545 to strike yesterday, the first day they were due for work. Negotiators for both sides said the main issues were class size, extra pay for extra work days and a stipend for materials and supplies.

In Michigan, 2,736 teachers in Pontiac, Marquette, Flint and Linden were on strike, idling more than 52,000 students