Education Secretary Terrel H. Bell, charging that American schools have become "academically flabby," yesterday urged school boards to require students to pass comprehensive exams before allowing them to be graduated from high school or promoted to certain key grades.

Bell, a former high school science teacher and state and local school superintendent, told a breakfast meeting with reporters here that in most American schools "upward movement is now automatic," a practice that he said has led to low standards and low achievement.

"There ought to be firm standards that students have to meet to move up the ladder," Bell said. "There ought to be no nonsense about it. Then you would have a lot more homework, because everyone would have to try to pass the tests, and a lot less 'Starsky and Hutch.'"

He said the standard-setting might be done by state education officials, such as the competency test requirements that 36 states, including Virginia and Maryland, have adopted over the past five years.He said he opposed national tests for graduation or promotion as an unwarranted extension of federal control.

But Bell said he strongly favored uniform tests, beyond those individual teachers might give, at the end of grades 3, 6, 9 and 12, the key divisions of the public school program.

"That way everybody, teachers and students, would be hustling," Bell said. "There would be a lot more creative tension in a system that has become academically flabby."

Many parent groups and school officials have suggested in recent years that increased testing might be an antidote to slipping academic performance, though the idea has drawn heated opposition from the national Eductaion Association, the nation's largest teachers' union.

Shirley Hufstedler, Bell's predecessor as education secretary, strongly questioned the value of standard tests. But Bell said he favored them.

"More and more, I'm coming to the conclusion that if you Don't have some tests, you don't have quality control," Bell said. "If you measure performance, you're going to improve it. If you don't measure it, then you get lazy."

In response to other questions he repeated his pledge to cut federal regulation of local school systems drastically as part of the Reagan administration's plan to replace categorical federal aid for specific school programs with block grants.

Although Bell said the details of the administration bill haven't been completed, he said he expects it to require that federal money to spent on low-income and handicapped students, although without the current elaborate federal rules.

On testing, Bell praised the program for system-wide promotion standards introduced in Washington by former superintendent Vincent Reed, whom President Reagan has nominated as assistant secretary for elementary and secondary schools in the Education Department.

Bell said he preferred local school systems to set their own standards rather than having statewide requirements, because often the state requirements have been too low.

State rules are better than nothing, Bell continued, "But ther ought to be something that stretches the capabilities of all the students, not just a low minimum." Bell suggested that some of the questions require essays, not just multiple-choice answers.

"The important thing is for every board to have some no-nonsense standards," he said, "and for everybody to know what they are."