More than 12,000 public school teachers from Oregon to New Jersey went on strike yesterday, joining about 8,000 colleagues who had already walked out, disrupting the start of fall classes for pupils in 11 states. In most of the disputes, the issue was money.

Among those launching strikes were about 6,000 teachers in 25 Michigan school districts, 1,425 teachers in Woodbridge, N.J., and 12,000 in Eugene, Ore.

The Michigan walkout increased the number of teachers on strike in that state to 10,000. There was no immediate estimate of the number of pupils involved in Michigan, although Lansing has 25,000 pupils and Saginaw, the second largest district hit, has 19,000.

In Pennsylvania, strikes were called yesterday in six school districts in the western part of the state. The new strikes raised the total to 10, affecting 2,326 teachers and 41,292 pupils.

Indianapolis teachers made good on their threat to leave classrooms in support of a new contract. A 13-hour bargaining session failed to resolve differences between negotiating teams at the Eugene, Ore., School District and teachers struck.

In Illinois, new strikes developed at four school districts including Springfield, where 900 teachers honored picket lines at 26 grade schools, four middle schools, three high schools and a special education center.

In Southinton, Conn., despite a state law making teachers strikes illegal, 434 of the town's 462 teachers voted to strike. The new school year for the district 8,000 pupils was not scheduled to being until today.

The strike by teachers in Woodbridge, N.J. affected 30 schools scheduled to open today. In Rutland, Vt., where the Rutland Education Association's 219 teachers have been on strike since Thursday, the salaries being paid substitutes, many of whom do not have teaching credentials, has become a complicating issue.

The substitutes are being paid $80 a day, more than three times the normal rate paid substitute teachers.