Six midshipmen at the Naval Academy -- including one woman -- face possible expulsion or milder punishment in what a spokesman called "by far the most major" sex scandal since coeducation began at Annapolis in 1976.

The woman, a 21-year-old junior, was allegedly tied to a bed by five male students, at her own request, and performed several sex acts with them while being filmed with an 8 millimeter camera.

The alleged acts constitute "conduct unbefitting an officer of the Naval Service," according to the school's regulations forbidding sexual acts in the dorm and "indecent or lewd acts" in general.

Capt. Clark Gammell, the academy's public affairs officer, said the academy has completed an investigation of the incident, at the order of Rear Adm. William F. McCauley, the commandant of midshipmen.

Some of those involved "are being recommended for separation [in this case, expulsion] and others will be the subject of disciplinary action administered under the academy's conduct system," ranging possibly from demerits to discharge, Gammell said.

Gammell, who declined to give details of the case, called this "by far the most major" sex-related disciplinary case since women were first admitted to the academy 4 1/2 years ago. Six other cases have been investigated, but none led to expulsions, he said.

"We haven't gone to battle stations because of it," Gammell said. "But we're naturally very concerned." Gammell said officials plan no changes in the dormitory arrangements or regulations as a result.

He would not reveal which students have been recommended for expulsion since the disposition of the case now rests with Secretary of the Navy Edward Hildalgo. Gammell said he expects Hildalgo to rule on the case before the inauguration of President-elect Ronald Reagan on Jan. 20.

The alleged incident occurred in Bancroft Hall, the 1,873-room coed dorm that houses all 4,500 students, including 289 women.

"The investigation has disclosed varying degrees of involvement on the part of the five male and one female midshipmen involved in the incident," Gammell said in a prepared statement. The recommended punishments are gauged according to involvement, he added. Gammell said no criminal charges grew out of the incident, since there was no evidence that force was used.

"The film was strictly an offshoot," said a knowledgeable source who indicated the five did not intend at first to film their acts. "They did one stupid thing and then compounded it. They got carried away."

Officials confiscated the film, but reportedly do not intend to develop it.

If expelled from the academy, the four senior men could face an automatic three-year tour of active Naval duty as enlisted men. The fifth male student, a freshman or plebe, would have no further Navy obligation if he is expelled.

As a junior, the woman could face two years of enlisted duty if she is expelled. She had applied before the incident to leave the academy because of medical reasons, and her request is still being processed, it was learned.