Six Mississippi state legislators will return today to work on pay raise proposals that could end a teachers' strike keeping 172,000 students out of class. They face the task of devising a plan acceptable to both houses of the Legislature, the strikers and Gov. Bill Allain. Meanwhile, one school district became the first to fire strikers who did not return to work as ordered. In dismissing 137 of 163 teachers, the Marion County school board acted under a state law that says teachers' contracts are void if they willfully abandon their jobs.