FOR NEARLY four years Terrel H. Bell has spent most of his time mediating between the education lobbies and the Reagan administration's ideologues. As secretary of education, he has suffered the slightly diminished status of a man in a job that his boss has declared unnecessary and a target for elimination. But Mr. Bell has used his limited means with considerable skill to deflect the administration's original intentions. At first, Mr. Reagan chiefly wanted to diminish all federal responsibility toward the schools -- except, of course, the asserted federal responsibility to push religious controversy into the classrooms. Instead, with the report last year of the National Commission on Excellence in Education, Mr. Bell has succeeded in broadening the debate within the administration at least to include some of the right questions about American schools.

The commission reflected the deep sense of disquiet over school standards that has been spreading in this country over the past decade. Requirements have often been allowed to go slack. High school students in particular spend too little of their time on school work, and too much of that time on trivial subjects. The report struck a public resonance that took the rest of the administration by surprise and gave political force to some good ideas -- impeccably conservative, in the most useful sense -- that became competitors to Mr. Reagan's original agenda for education.

But the original Reagan agenda remains very much alive. Federal aid for the education of disadvantaged children was cut sharply in the first Reagan administration, and attempts to cut it further may be ahead. The Justice Department's loss of interest in desegregation isn't going to be reversed. The president continues to advocate disruptive innovations such as the prayer amendment and the tuition tax credit. Secretary Bell's commission observed that only the federal government can define and develop the national interest in education. Mr. Reagan has responded by offering to send a teacher into space on the shuttle.

Mr. Bell has found his job wearing, understandably, and is resigning. Since Mr. Reagan has apparently postponed his plans to abolish the Department of Education, he needs to find a successor. There are a couple of questions that any candidate will want to ask. Is the next secretary going to have to spend full time fighting off the the religious issues, and the assaults -- like the tuition tax credit -- on the basic concept of public education? Or will it be possible to devote serious effort to strengthening the schools? There won't be much federal money for it. But even on a low budget, a Cabinet job allows a vigorous advocate to exercise real influence. Mr. Reagan's choice of the next secretary will be an important indication of his intentions for social policy in his second term.