When the seven-member Prince William County School Board meets Wednesday to approve or reject Superintendent Edward L. Kelly's proposed high school attendance boundary changes, board member Donald A. Chendorain will likely cast the swing vote.

According to interviews, the other six members appear to be evenly split on the issue, which has widened a chasm on a board known for its lack of unity. The boundary proposal would affect high school students in the eastern end of the county and has stirred considerable debate between neighborhoods there.

Board Chairman Gerard P. Cleary, Vice Chairman Maureen Caddigan and member George Mullen appear to support Kelly's recommendation. Ilona Salmon, Patricia L. Cusey and Odis Price favor other options, they said in interviews.

Chendorain says that he is being lobbied heavily by callers in the neighborhoods affected but that he has not made up his mind.

The issue has apparently strained relations between Kelly and Salmon, whose Occoquan District would be split between two high schools, Gar-Field and Woodbridge, if the superintendent's recommendation is followed.

Salmon and the Gainesville District member Cusey left a board meeting last week just before the remaining members voted confidence in the superintendent, 5 to 0.

At last week's meeting, Salmon argued vociferously against the superintendent's recommendation, known as Option D, and in support of another proposal, called Option A. Under Option A, students in her district would remain together at Woodbridge High School instead of being split between two schools. Price has also said he favors Option A.

Cleary, who represents Woodbridge, has said he cannot back Option A, which would split his district between Woodbridge and Gar-Field.

Salmon argues that Option D provides less relief to crowded Woodbridge High, which would serve 2,944 students in 1990, or 250 more than its capacity.

Kelly and his staff defend Option D with predictions of population growth in areas within the projected boundaries. They argue that the boundaries of Woodbridge would comprise a no-growth area and that Woodbridge High's population will decline after peaking in 1990.

The students who would be sent to Gar-Field live in areas that are still being developed, Kelly says. Under Option D, Gar-Field would be 200 under capacity in 1990 and could accommodate growth.