Tennessee's teachers are buzzing--not all of them happily--over Gov. Lamar Alexander's crusade to improve the state's public schools by giving substantial raises to the best teachers.

Ordinarily the governor's proposal wouldn't have much of a chance because it smacks so much of the "merit pay" idea of which teachers' unions, including the Tennessee Education Association, are almost automatically suspicious.

But the second-term Republican governor, who describes the bill as the most important legislation he will introduce in his eight-year tenure, has been crisscrossing the state, talking --and listening--to teachers, administrators, politicians, civic leaders and parents about his plan. Members of his Cabinet and the bipartisan educational task forces of both houses have been doing the same thing, and the sense here is that the plan has won a significant number of converts.

The bill he introduced last Thursday is, according to his staff, "a framework for discussion and debate, and was presented as such."

At the heart of Alexander's plan are two points: bonus pay for outstanding teachers and periodic reevaluation of all teachers. Its unspoken rationale is the declining attractiveness of teaching for the brightest college students. One recent survey revealed that education majors ranked 17th in math ability and 14th in English ability among 19 fields of study, and that the trend is worsening. What has happened, of course, is that teaching has remained largely a woman's field, and the brighter women are going into other, more lucrative careers as sex discrimination declines.

Alexander, who believes his plan will attract more high- ability students into teaching and keep them there, is counting on substantial bipartisan support. He is Tennessee's first Republican governor since 1970, and only the fifth since Reconstruction, in this heavily Democratic state. "Our public schools will be among the best in the country because many of our teachers would be among the best and the best-paid in the country," he said. "Tennessee would lead the nation."

His plan calls for a four-tier structure, beginning with "apprentice" teachers, who would be paid generally along existing scales. After successful completion of their apprenticeships, teachers would be given renewable licenses as "professionals," along with a raise in pay. Then would come "senior" teachers, who would work an 11-month year and be paid a 30 percent bonus, and "master" teachers, who would work 12 months, both as classroom instructors and as trainers of less-experienced teachers, in exchange for a 60 percent bonus.

It is expected that after eight years, nearly two-thirds of all teachers would be earning bonus pay. But a third wouldn't--and, because of the reevaluation requirement, some of that third might be terminated. Thus substantial teachers' union opposition can be expected.

Alexander's plan is a bold attempt to resolve the dilemma confronting school districts across America: teaching pays too little to attract the brightest college students, with the result that teacher quality--and public education generally--has been declining. On the other hand, it is hard to sell the notion of granting substantial pay raises to a profession that, in the view of taxpayers, is performing poorly.

The toughest debate will be over how to judge-- and who will judge--which teachers deserve the top classifications: the problem that has stymied most merit-pay proposals.

Still there's a good deal to be said for a proposal that would reward teachers for demonstrated on-the-job competence. We reward them now for accumulating postgraduate credits and years on the job, without regard to how well they do what they are paid to do.

Few Americans are happy with the results.