When Alexandria public schools reopen Sept. 6 to an expected 10,304 pupils, many of the system's 15 elementary and junior high schools and its one senior high will start classes more than an hour earlier or later than last year, a change that promises to scramble morning routines in households throughout the city.

"It will certainly be the greatest change facing parents this school year," city School Board member Sandra Lindsay said. "It will affect most of the schools."

The school system moved to new opening times because the number of Metrobuses is expected to be drastically cut when Alexandria becomes part of the Metrorail service area early next year, and the city must shift 6,400 students a day from Metrobuses to school buses.

"We had to rearrange our school hours in such a way so we could get more students per bus," said John (Jay) Johnson, chief of school transportation. "That would reduce the number of buses we had to purchase."

This week, the first of a dozen new $21,000 school buses are to arrive in Alexandria, boosting the school fleet to 65. Without the juggling of school opening hours, Johnson said, as many as 35 new buses would have been needed.

The first class period at T.C. Williams High School will begin at 7:30 a.m.

Hardest hit by the rescheduling will be kindergarten and elementary school pupils when their classes begin later than the time their parents must leave the house for work. To help solve that problem, school officials are establishing expanded day-care programs for an estimated 180 children who may be affected.

Donald E. Dearborn, assistant superintendent for elementary and special education, said parents, depending upon their income, will be charged from $2 to $20 a child a month for the service that will provided be in Jefferson-Houston, William Ramsay, Cora Kelly, John Adams, James K. Polk and Mount Vernon schools.

Parents will be responsible for bringing their children to the centers where dance, games, arts and crafts and drama will be supervised. The programs will begin at 7:30 a.m.

Older students appear to like the new schedule because it means the school day will end earlier.

"It's a great idea," said Robert Lusk, 15, who will start 11th grade next month at T.C. Williams High. "It will give me more time to ride my bike. And I won't mind at all the 7:30 classes because I get up early anyway."

Sophia Brock, 17, has only one problem with the new transportation plan. "We have to ride them 'cheese buses,' " she said, referring to the bright yellow school buses. "I don't like that--three to a seat."

Other changes that will mark the start of this school year include the introduction of mandatory instruction of ninth graders in family life, or sex education.

School Board officials said recently that this year the board will be consider whether sex education courses should be taught in elementary school.

Administratively, Superintendent Robert W. Peebles announced two new elementary schools will have new principals this fall.

Mary L. Beach will replace Lucy Kirby, who is retiring after 30 years in city schools, as principal of Douglas MacArthur Elementary School.

Beach, a reading and language arts specialist, was principal of Brennen Elementary School in Columbia, S.C., for nine years.

Richard J. Aubry, an Alexandria native, has been assistant superintendent for administration of Manassas Park schools. He will replace Frances S. Beasley, who is on a leave of absence, as principal of Charles Barrett Elementary School.