A 12th grade guidance counselor at Duval Senior High School in Lanham has been placed on administrative leave with pay while school officials try to sort out how and why more than 10 seniors received grades for classes they never took or were given credit for classes they failed, Prince George's County school officials said yesterday.

The altered grades and credits allowed at least nine seniors to participate in graduation exercises at the school on June 9, minutes after their grades were questioned by school officials. Five of those students were denied diplomas a week later, a school spokesman said.

Other students were given diplomas because they had completed enough requirements without the altered grades, according to school spokesman Brian J. Porter. He said still other students whose transcripts were altered did not meet graduation standards anyway and were not commencement candidates. Porter said he did not know the total number of students involved or whether this has happened in previous years.

Porter said that only the school counselor appeared to be implicated and that officials are still trying to determine why the grades were changed.

"There was no indication that there was any kind of collusion, any kind of payment for grades," Porter said.

School officials declined to confirm the counselor's name, saying that personnel matters are confidential.

But one of the students, Sammy Brown, said yesterday that counselor Robert Jones granted him credit for courses in art and literature that he never took and gave him graduation credit for a part-time job as a grocery store cashier.

Jones could not be reached for comment yesterday. School officials said he has been on leave with pay since June 5.

Brown said that in May and June 1986, Jones assigned him independent study projects, which included drawing pictures, so that he could enter the 12th grade in the fall.

"He put a lot of changes on my card," Brown said. He defended the arrangement, saying Jones is genuinely interested in helping troubled students.

"I did the work. It's not like he gave us something for nothing," Brown said.

The problems at the school at 9800 Good Luck Rd. in Prince George's County came to light just days before the graduation of more than 300 Duval seniors.

According to Porter, a teacher at the school became suspicious of the graduation plans of two academically troubled students, prompting a review of their records.

"What they found was that the students had been given credit for courses they had not completed," Porter said. In some cases, failing grades had been changed to passing grades as well, he said.

School officials subsequently pored over the records of each senior and were still reviewing them on graduation day. Some students were informed of the discrepancies "while they were getting into their caps and gowns," Porter said.

The five students who have yet to receive diplomas need from one-half to one full credit of course work to graduate.

School officials said the students should be able to complete the work in summer school.

Brown, 20, said he did not graduate this spring because of other courses he failed during the school year. It may take him another year to get his diploma, he said.

"If they hadn't found out and I hadn't failed those {courses}, I would have a diploma right here on my mantel," he said.