Alaska is the pacesetter and South Dakota brings up the rear when it comes to salaries for public school teachers, the National Education Association says.

Classroom teachers now earn an average of $28,031 nationally, the union said Thursday, with salaries ranging from $40,424 in Alaska down to $19,750 in South Dakota.Alaska is one of nine states, plus the District of Columbia, where the typical teacher earns more than $30,000 a year. Only South Dakota fell below $20,000.

The average classroom teacher, with 15 years of experience, got a 5.6 percent pay raise last year that boosted his or her salary by $1,477 from the 1986-87 level of $26,554.

The roster of top-paying states includes such populous ones as New York, California and Michigan, and that will help push the national average teacher salary closer to $30,000 in the year ahead.

The teachers group did not estimate how much salaries will rise this fall, but if teachers get another 5.6 percent increase, they will be earning $29,600 for the 1988-89 school year.

Teacher salaries have risen sharply in the 1980s amid widespread concern about lagging standards in the classroom and waning interest among college students in entering the profession. Recently, however, with starting salaries topping $20,000 in some places, interest in teaching has picked up.

Secretary of Education William J. Bennett, in a critique last week on the impact of school reforms, said teacher salaries have risen 20 percent after adjusting for inflation since 1980-81.

But the NEA said that over the past decade, including the steep inflation years of the late 1970s, teacher salaries are up only 7.3 percent after inflation.

The union said there are 2,275,209 classroom teachers in the public schools, 25,251 more than in 1986-87.

The entire instructional staff, including principals, supervisors and specialists, numbered 2,589,148. That was up by 30,190, or 1.2 percent. The average salary for all instructors was $29,203, up 5.3 percent from $27,722 last year.

Most teachers work on nine- or 10-month contracts.

Eighteen states and the District of Columbia pay teachers more than the national average; 32 states pay less.

NEA researchers said the $28,031 figure is an estimate based on salary data compiled from all 50 state.

Although only an estimate, the NEA salary figure is widely cited, including by the U.S. Department of Education, as the most accurate indicator of how much school teachers earn.