Prince George's School Superintendent John A. Murphy told a congressional panel yesterday that the school system plans to back up its claims of improving achievement by guaranteeing that its high school graduates are ready to hold jobs.

If graduates are found lacking by employers, Murphy said, the school system will re-educate them free of charge.

"If we haven't done the job, then we'll redo it at our own expense," Murphy told a subcommittee of the Joint Education Committee that is examining the state of the American work force.

The idea of a guaranteed education has been employed in California and Michigan, but Prince George's is the first school system in the Washington area to offer the guarantee to employers. The idea was spawned locally from a study by a school business advisory council concerned about the quality of applicants in the Prince George's job pool.

The educational guarantee is the latest of a variety of tactics and ventures that schools and businesses have undertaken together to improve education and provide businesses with better-prepared applicants.

Murphy said the county expects to issue the "guaranteed diploma" to all students in addition to the regular high school diploma. The first certificates most likely will be issued to 1989 graduates.

The guaranteed diploma would list specific skills, apart from those covered by regular graduation requirements, which county students should have mastered.

Louise F. Waynant, associate superintendent for instruction, said officials and area business leaders are drafting a list of skills considered vital for all employes, whether they are working in fast-food restaurants or high-tech firms. Skills included on the list are not the typical vocational skills of typing and accounting but include such "career skills" as computing simple interest, translating data from charts and graphs and analyzing inconsistencies in written material.

"We'll give {the} student a guaranteed certificate that will tell the employer that the youngster is ready to go to work," Murphy said.

Under the current plan, any business could use those specific sklls as a checklist to evaluate new employes. If a Prince George's graduate comes up short in any area, the county would provide free evening or weekend classes through its Adult Education Program, so the students can continue working. Courses usually cost about $30 each.

Throughout the area and nationally, as more schools opt for providing special diplomas or seals that certify students in basic skills to advanced achievement, some of the value of the regular high school diploma has come into question.

High school graduates receive diplomas based on a general completion of a set of standards involving the kind and number of courses a student has taken. State-mandated minimum competency tests, such as those in Maryland, are meant to assure but do not guarantee real mastery.

Prince George's schools spokesman Brian J. Porter said the county plan "is a step above what the state requires." Under state regulations, he said, "You could graduate with straight Ds."

Still, some sectors of the education community take a skeptical view of separate guarantees.

"There's a general understanding that a {high school graduate} will be able to read and write and speak within a reasonable level of fluency. I'm not sure one needs to add pieces to it," said Vito Perron, vice president of the Carnegie Foundation for the Advancement of teaching and an expert on high school education.