A federal judge here ordered the District government yesterday to reinstate and provide full back pay, seniority and other benefits to a school bus attendant fired after one unconfirmed test indicated marijuana use.

Juanita M. Jones, a 49-year-old grandmother who had been praised as a model employe, was summarily fired in August 1984 after being tested as part of a mass screening of school transportation workers. Jones, who had worked for four years as an attendant on a bus for handicapped children, denied using drugs and submitted two urine samples that showed no evidence of drug use.

Granting summary judgment for Jones in the bulk of her case, U.S. District Judge Louis F. Oberdorfer found that demanding Jones' urine sample without any "particularized probable cause" to suspect she was using drugs constituted an unreasonable search and seizure in violation of the Fourth Amendment to the Constitution.

He suggested, however, that school bus drivers or mechanics responsible for school bus maintenance "might reasonably expect to be subject to urine and blood tests" without probable cause.

Oberdorfer also ruled that discharging Jones without confirming the results through a second testing method, as the manufacturer recommends, was "arbitrary and capricious" action that violated the school system's own rules.

In addition, Oberdorfer said, firing Jones without any hearing to determine whether the test was of her urine and had been properly confirmed, violated her constitutional due process rights. He said, however, that "the public's vital interest in the safety of schoolchildren" might permit temporary reassignment or suspension before tests are confirmed.

Assistant Deputy Corporation Counsel Michael E. Zielinski declined to comment on the ruling.

Oberdorfer's order required authorities to expunge the termination from Jones' record.

As part of the mass screening program instituted in August and September 1984, 27 other employes were fired, some of whom have sued. Although Oberdorfer's order does not specifically apply to them, "it certainly should," said Arthur B. Spitzer of the American Civil Liberties Union, which represented Jones.