The 123 students at Washington's Ballou High School, whose aptitude test scores were nullified by a national testing service, were distracted by everything from loud noises to the presence of a nervous mouse, said school system officials.

An investigative team of school officials, supervised by School Superintendent Floretta D. McKenzie, also noted that the students on test day last October were bothered by several announcements on the public address system and by a timer that sounded prematurely when the students actually had more time to work. A mouse also ran across the room, scaring some students.

Ballou counselors, who subsequently felt it was impossible for the students to concentrate on the exam, then let 100 students break a testing rule by having them complete the exam the following day, said school officials.

When a parent reported the infraction to the New Jersey-based Educational Testing Service, which devises and scores the standardized tests, the agency rejected the students' scores. ETS, however, has agreed to let the students take the test again in 1985.

The PSAT is an indicator of how a student will perform on college entrance tests and is used in selecting finalists for a national scholarship program.

The team was created to look into the test in the hope of avoiding such problems in the future.

"We're going to ensure that an event like this is not going to occur again," said Ballou Principal Helena Jones. "We look forward to the development of new strategies to implement for the betterment of the students. There's no way in the world this can occur again."

Ballou counselors told the team that they would "conduct a pre-orientation program for students, delay the opening of school and explore the possibility of testing on Saturday and using the . . . mini-gym" before next year's test.

The team's report also noted that much time was lost because counselors found it difficult to understand the instructions they were supposed to read to the students.

The report also made several recommendations for improving counseling services at Ballou and other city high schools.

Some of them include a training program for counselors in the administration of the PSAT and SAT test; establishing an on-going test orientation sessions for students, and hiring more counselors at Ballou, which only has one for every 500 students.