By 7:20 a.m. Tuesday, more than 150 school buses will begin crisscrossing Loudoun County's 517 square miles to carry about 10,000 students back to class in the county's 31 schools.

To ensure safe journeys throughout the year, intensive school bus safety training will lead the list of new programs for the 1987-88 school year, school spokeswoman Molly Converse said. In addition, bus safety will be reemphasized at all grade levels throughout the school year.

Officials are expecting more than 13,800 students to enroll this year, an increase of 540 from last year. The 4 percent increase in Virginia's 16th largest school system is attributed to booming residential development in the eastern county and in Leesburg.

To handle the increase, voters approved a $10.2 million school bond in March that will allow the county to build and equip two elementary schools, one in the Countryside subdivision and one in Leesburg. Officials say, however, that another elementary school, a middle school and a high school will also be needed to accommodate the 4,000 new students expected to swell school rolls in the next five years.

Converse said new math textbooks will be introduced in grades 1 through 12 and the elementary art curriculum will be strengthened with a three-year program that will bring certified art instruction into 13 of the county's 21 elementary schools. Until this year, classroom teachers and parent volunteers taught art in kindergarten through fifth grade.

Middle school students will be offered a new honors program in math, science and language art, Converse said.

AIDS education will be part of the health and sciences program for seventh and eighth graders.

In addition, each high school will have 24 new computers. Juniors and seniors will also choose from new humanities courses in religion, philosophy, the arts and architecture.Writing Lab in Fauquier

In Fauquier County, school officials expect 8,098 students to enroll, an increase of about 300 over 1986.

Assistant Superintendent James Brumfield said the increase is due to growth in the south and central parts of the county as well as a higher birth rate. In an attempt to prepare for continued growth, the Fauquier Board of Supervisors sent a $30 million bond referendum to the voters in March designed to fund the construction of an elementary school and a high school. It was defeated.

Said Brumfield, "We're preparing to go to the voters, but we won't be ready by Nov. 3." Twenty trailers will handle the overflow on school grounds throughout the county, he said.

Among the changes returning students will find this fall are a composition writing lab for journalism and English students designed to enhance writing skills, and an expanded printing program at the vocational-technical annex at Fauquier High School, according to instruction Superintendent Dorothy Standridge. Prekindergarten in Stafford

Stafford County will implement a prekindergarten this year, designed for children with late birth dates (November and December) who need additional preparation.

According to school spokeswoman Donna Krueger, the program is modeled after the state's pilot program, which was implemented in Chesterfield County recently. Krueger said Virginia officials approved the program because many children with late birth dates are not ready for kindergarten and are required to repeat it. "To fail when you're that young can be devastating," Krueger said. "This is a hands-on development program that will prepare the younger children for kindergarten."

Elementary and middle school teachers will participate in a program that will help them understand their attitudes toward students and how their behavior affects student performance, Krueger said. The program, called Teacher Expectations Student Achievement, will ask teachers to observe one another at work, she said.

Teachers in grades 1 through 6, who have been trained at George Mason University, will participate in a new writing program that will help pupils improve their writing and editing skills in all subject areas, Krueger said.

Student enrollment this year is expected to reach 11,150, up 550 from last year, Associate Superintendent Norval Waugh said. To handle the increase, a third grade and a kindergarten were added in late August, Waugh said.

The new Widewater Elementary School, slated to open in two phases, will accommodate 750 students. Waugh said 600 pupils are already enrolled. Third and fourth graders will enter before Nov. 1 and kindergarten through second grade pupils will enter in January.

Stafford voters approved a $30 million bond referendum in August designed to build two new elementary schools in the more populated northern end of the county and to repair five schools.