Cpl. Jerry D. Rousseau, a boyish faced 21-year-old marine charged with raping and murdering an American school teacher in Morocco, pleaded guilty yesterday to a charge of unpremeditated murder.

The surprise plea by Rousseau, whose case had caused a jurisdictional dispute between the American and Moroccan governments, was entered at the Quantico Marine Base where Rousseau has been held since he was returned from Africa. He was assigned there as an American embassy guard.

Under a plea-bargaining agreement accepted by military judge, Col. Robert E. Switzer, charges of rape and premeditated murder against Rousseau were dropped in exchange for the guilty plea.

Rosseau was charged in the June 3 death of Dorothy Layton, 39, of Cuba. Mo, who was raped, beaten, strangled and stabbed in her apartment in the casbah shortly before she was to return to the United States to rejoin her husband.

At yesterday's hearing, the Farmington, N.M. native showed little emotion as Col. Switzer told him that "this court finds you guilty of specification," a murder charge in the Uniform Code of Military Justice. Rousseau could face a penalty of life-imprisonment for his guilty plea. A spokesman at Quantico said yesterday Rousseau will be sentenced today or Saturday.

Earlier this month, Col. Switzer ruled that the Marine Corps had the authority to try Rosseau despite defense arguments that Rosseau's enlistment should be declared fraudulent. His recruiting officer had told him to cover up an arrest for public drunkenness and his admitted use of marijuana, Rosseau had claimed. Marine Corps policy is not to accept recruits who have smoked marijuana.