A 16-year-old Clinton youth was acquitted yesterday of charges of assaulting a Landover Mall security guard and resisting arrest, but found guilty of trespassing at the shopping center last November.

Prince George's County Circuit Court Judge David Gray Ross set sentencing for John Thompson Jr., of 5804 Terence Dr., for April 25. Maximum punishment would be committment to training school until he is 21.

In a separate trial, Ross last week dismissed similar charges against Thompson in connection with an alleged attack on another security guard.

The charges stem from a Nov. 27, 1984, incident in which Thompson and his uncle, Eugene Darnell Thompson, 18, claim they were beaten and kicked by mall guards before being arrested. Another man, Mark Gibrau, 20, of Landover, said he was grabbed and choked by guards later that night.

The three guards involved in the incidents were arrested by Maryland State Police last week and charged with assault and battery in the cases.

One of the guards, Gary F. Byrd of Capitol Heights, is still working at the mall, just off the Capital Beltway in the north end of Prince George's County, according to state police spokesman William Tower. Tower said the two other guards, David Kevin McCloud, 20, of Silver Spring, and William Dean Ehresman, 24, of Upper Marlboro, are no longer employed there.

Charges against Eugene Thompson and Gibrau are still pending.