The Fairfax County supervisors came to the rescue of the county's school bus service last night, providing assurance that $400,000 in budget funds will be restored to save thousands of students from having to walk as far as 2 1/2 miles to their schools.

The election-minded Board of County Supervisors had been the target of numerous complaint from parents angry that their children would be forced to walk long distances to school because of a cut in the school budget.

The decision to cut back bus service was made by the school board as a means of absorbing part of a $1.4 million budget cut dictated by the supervisors.

Last night's good news came from Board of Supervisors Chairman John F. Herrity, who told the school board at a meeting before a large group of parents that the supervisors are willing to restore money for busing at their meetin next Wednesday.

The decision last month to increase the distance intermediate school students must live from their schools to be eligible for busing from 1 1/2 to 2 miles and to increase the distance for high school students from 1 1/2 to 2 1/2 miles was seen by the school board as a means of saving 103,000 gallons of gasoline.

But Herrity said last night he doesn't believe increased walking distances will save money because "parents will drive their children to school."

Also on hand to reassure the school board was Supervisor Audrey Moore, who said that longer walking distances would pose "serious and dangerous" conditions for students who live in the Annandale District, which she represents.

Both Herrity, a Republican, and Moore, a Democrat, are up for reelection this fall.

The school board's vote to reverse its decision to cut busing and restore the 1 1/2-mile maximum for walking to school was 7 to 1, with board member J. Roger Teller of Mason District in the minority.

Herrity and Moore also told the board that the supervisors stand ready to make up losses the school budget would incur if federal aid to areas with large numbers of federal workers is reduced. Congress is threatening to cut these impact aid funds.

In other action, the board voted 5-3 to leave intact its controversial year-old grading system.

At present, Fairfax has a "six-point" system. For example, a grade score of 94 to 100 points is valued as an "A"; a grade between 87 and 93, as a "B" and so on.

An advisory committee of parents, students, teachers and administrators had suggested that the grading system be changed to the more traditional 10-point system. The board, after a two-hour discussion and several interim votes, decided against the change.

The board also voted to reduce from 10 to five the number of excused absences allowed students per quarter. Students who miss more than five classes in a quarter would receive a failing grade in that course.

Last night's meeting was the last for School Superintendent S. John Davis, who will take up his new post as Virginia's Superintendent of Public Education on Wednesday. The school board has launched a nationwide search for a replacement.

The school board has set a deadline of Aug. 24 for submission of applications for the $51,000 post.