School doors began opening across the country this week with fewer teacher strikes than at any time in recent years. But a new, critical problem faces American education: a shortage of certified classroom teachers.

The general calm on the labor front -- only about 2,000 teachers had walked out as of yesterday -- is attributed to salary increases in almost every state, a more favorable public mood toward education, and new recognition by teachers that strikes have often generated as much ill will as they achieve in benefits.

The shortage, on the other hand, is the result of complex demographic and labor market trends, including the aging of the current teaching force, a "baby boomlet" filling classrooms, and declining numbers of college students entering the teaching profession.

Some school districts have resorted to stopgap hiring measures, including allowing those with bachelor degrees but no teacher training to shortcut the normal process and teach after receiving emergency certificates.

In Los Angeles, for example, a school district recruiting ad reads: "Want to Teach but Have No Credentials? Relax!"

Other districts facing shortages have gone overseas, particularly for much-needed bilingual teachers. New York City recently hired teachers from Madrid, while Houston advertised in Mexico City newspapers. Washington area schools have been an exception to the trend, with superintendents reporting no problems finding properly certified teachers.

Emergency measures being pursued in states with shortages are opposed by the National Education Association, which called a "back-to-school" news conference yesterday to highlight the teacher shortage and urge that all certified teachers display framed teaching certificates on classroom walls, much as doctors and lawyers display their degrees.

"These so-called emergency and alternative certification policies make a mockery of the education-reform movement," said Mary Hatwood Futrell, president of the 1.7 million-member NEA, the nation's largest teachers' union. "The politicians have called for stronger teacher standards. Now they want to lower them."

Futrell, outlining her steps for dealing with the shortage, urged states to establish interstate reciprocity pacts to allow teachers certified in one state to move to a state experiencing a shortage without losing benefits and experience levels. She also said the Department of Education should establish a national clearinghouse to help states locate new teaching graduates.

Futrell said many qualified teachers left the field when they were unable to find jobs during the teacher surplus a decade ago. States should try to locate them and press them back into service, she said.

Those steps, along with increasing teacher pay, would solve the shortage, she said. Increased education funding has raised the average beginning teacher's annual salary above $16,000, although the NEA says that figure should be raised to $24,000.

Teacher pay increases have averaged about 7.5 percent nationwide for the current school year, as inflation runs at about 4 percent. The largest increases have been provided by southern states where teacher pay has traditionally been lowest and public support for public education weakest.

The higher salaries, along with lower inflation, have been cited as principal reasons why the current school year has begun with fewer teachers on picket lines.

Nationwide, walkouts by about 2,000 teachers yesterday kept more than 52,000 children home in Michigan, Illinois, Ohio and Pennsylvania.

Last year, in the first week of school, more than 100,000 children were kept home because of walkouts by 7,300 teachers.

"State legislatures are becoming much more generous. They are the main friends of education today," said Scott Widmeyer, spokesman for the 600,000-member American Federation of Teachers, which has led sometimes-bitter strikes in many urban school districts.

"There's a better mood on the part of everyone," he said. "It seems like the goal is to avoid controversy where you can."