Departing Education Secretary T.H. Bell spent his term fighting an eroding, unabated conservative onslaught. Insiders say it finally wore him down and out.

In a second term, he would have faced more battles with Office of Management and Budget Director David A. Stockman, who has made it clear that he is going after federal education spending with a vengeance.

His service as a World War II Marine nothwithstanding, Bell is not disposed to fighting, especially if the battles are political and intramural.

His dedication to improving national education, rather than tuition tax credits and vouchers, proved his undoing.

A high-ranking official of the Department of Education said President Reagan's response to Bell's letter of resignation was barely more than a "courtesy thank you note."

Reagan's 1980 campaign pledge to eliminate the Department of Education had become a battle cry for conservative Republicans. They wanted action and results. They blamed Bell for putting abolition of the Department on hold and faulted him when the party platform did not mention it this year.

Reagan had promised to stay the course, but conservatives said they felt that Bell had fouled the plan. It was not so much that he was increasing federal spending levels -- he wasn't -- but he wasn't slashing spending either.

In his swan song Thursday, Bell took note of his differences with conservatives, saying, "I've always felt that there's a significant role for the federal government in education . . . enhancing the capacity of the state and local entities to meet the needs of education. And in addition to that, I feel we have a leadership role . . . .

"I've always felt that I would be able to persuade my critics that the view and the position and the stance that I've taken is the correct one, and that to move in the radical direction of abolishing all of our programs and dramatically withdrawing all federal concern and support of education would be a mistake . . . . I've not been successful in doing that . . . . "

Bell had been U.S. Commissioner of Education in 1974-76. (The Department of Education was created in 1980 at the request of President Jimmy Carter.)

Bell was Commissioner of Higher Education in Utah when he was called to Washington. He had continued to teach education at the University of Utah while he held the post.

By the time he announced his resignation last week, a once skeptical education establishment mourned the loss of a friend.

"He's done a lot to put the spotlight back on education at all levels," said Robert H. Atwell, acting president of the American Council on Education. "He had the unhappy task of carrying out policies for an administration not disposed to education, but he did it with class.