The Montgomery County School Board has reversed a decision by Supt. Charles M. Bernardo and will permit a mentally retarded 12-year-old to attend a public school.

In a letter addressed both to the child's father, Fred Ederer, and Bernardo, the board said it is ordering placement of Judith Ederer, who is severely retarded, in an appropriate public school program starting next month.

The Ederers had appealed Bernardo's decision to exclude their daughther from the Montgomery public school system. They argued at a hearing in June that Judith would benefit more from being in a community public school that has programs for the severely retarded than from the institutional setting at the state-operated Great Oaks Center in Silver Spring, where she now lives and goes to school.

Bernardo had said that since Judith was getting adequate schooling at Great Oaks, she should not have high priority for one of the new spaces at public schools that have problems for the severely retarded.

School board member Marian Greenblatt said the fact that the board approved the appeal illustrates a trend for local governments to assume responsibility for educating the mentally handicapped.

"The state is backing away from its responsibility," Greenblatt said."They want this to be the function of local governments. People don't realize that the implications of this are nationwide." Greenblatt said that putting more children in the special education programs run by the county could necessitate cutting back money for other educational programs to finance a larger special education staff.

Board member Elizabeth Spencer, who voted against the order, said she agreed with its intent but thought the board's action was "promising something I'm not sure we can deliver." Spencer said, "I'm just not sure we can do this without adding another teacher or bumping another child. I also don't know if there will be similar children wanting to go into the public schools."

More mentally handicapped children in the public school system's special education programs may only mean shifts in staff from one program to another, according to Bill Porter, who oversees special education centers for county schools. Porter said they would never remove a child from the system to let another one in.

Board president Herbert Benington said the board will write a formal opinion in the Ederer case in the next two weeks. Benington said the board would decide individually any future cases of mentally retarded children who seek enrollment in a public school program.

The Ederers, who said they were delighted with the board's decision, also have an 8-year-old mentally retarded daughter, who is now in a community preschool program. A similar appeal hearing on whether or not she can be placed in a public school program will be held later this month.