Edward M. Elmendorf, who has been in charge of college student aid programs for the past four years, has resigned as an assistant secretary of education to join a high-technology development firm based in Ann Arbor, Mich.

In a letter sent yesterday to his staff, Elmendorf said he is leaving after four years to become a corporate vice president of Wood and Co. Inc., which is developing properties in Ann Arbor, Tampa and Leesburg. The new job, he said, will allow him to become involved with a new development venture that will bring together the private sector and the higher education community.

Elmendorf's unexpected resignation, effective June 15, will leave the Office of Postsecondary Education with two key vacancies, since Benjamin A. Alexander previously resigned as deputy assistant secretary for student financial aid.

The resignation also will give Secretary William J. Bennett another opportunity to place his people in department jobs.

In a meeting with reporters yesterday, Elmendorf said he is leaving the department with a sense of accomplishment, after presiding over a crackdown on people who defaulted on federal student loans and after increasing his office's use of computers.

Asked what advice he would give his successor, Elmendorf said, "Listen, at least for the first six months, and learn. It's easy to make snap judgments . . . . I would also advise him to treat people as people." NRC Bill Obstacle Cleared

* Rep. Dennis E. Eckart (D-Ohio) dropped his objections yesterday to the Nuclear Regulatory Commission's fiscal 1986 authorization but said he intends to continue to fight the agency's decision last week to hold more of its deliberations behind closed doors.

Eckart, supported by fellow Democrats, offered an amendment to the commission's authorization bill before the House Energy and Commerce subcommittee on energy conservation and power.

Eckart then yielded to a request by subcommittee Chairman Edward J. Markey (D-Mass.) that he withdraw the amendment because the authorization bill was "not the proper forum" for his objection.

The NRC voted last Thursday to change the definition of "meeting" in its regulations so that commissioners can meet privately without violating the Government in the Sunshine Act.

George Rasanen, an Eckart spokesman, said Eckart withdrew his amendment on the condition that the subcommittee act quickly on a proposal he intends to submit soon that would reverse the NRC vote.