The signs are everywhere.

At Kettering Middle School, the front office counter was stacked earlier this week with teacher "care packages": stacks of copy paper, "cut slip" pads and yellow sponges for wiping blackboards and desks.

Nearby, at Kettering Elementary School, Principal Lucy Marr busily registered new students. And at Thomas Pullen School, workers recently put the finishing touches on new dance, music and drama studios.

But the sign at Pullen's front entrance, purple letters two feet high, said it all: "Welcome Back."

Prince George's County opens its schools to 102,500 youngsters on Monday, ushering in a year that will introduce even more options and programs in the school district that has come to call itself a "system of choices."

As part of its continuing desegregation effort, the county will open new magnet programs, including the Center for Creative and Performing Arts at Pullen. More extended day programs and new classes for preschoolers will mark the 1987-88 school year as well.

Perhaps more important, however, the school year begins two years closer to the goal Superintendent John A. Murphy has set for the nation's 16th-largest system: scoring in the top quarter nationally on standardized tests by 1990.

For the last four years, it has been on the right track, as students have made steady improvements on functional and standardized tests. Student discipline has improved, measured by a 35 percent decrease in reports of vandalism and disorderly conduct.

"We're in very healthy shape," said Murphy, adding that he hopes educators and parents will work together to maintain the momentum of higher achievement. "The continuing effort to move ahead is the single most important goal."

Some of the improvements have to do with teachers. As of last week, the county had hired 357 teachers from among more than 2,000 candidates.

After commuting to teach in Anne Arundel County for seven years, Denise Johnson, who lives in Landover, will teach a sixth-grade class this fall at John Bayne Elementary.

"I'll be working in my own community," she said.

The county's popular magnet school program, designed as a desegregation tool to attract whites to predominantly black schools, opens four more programs. Beltsville Elementary and Martin Luther King Jr. Middle schools will open as new academic centers, offering accelerated science and mathematics programs, Latin and foreign languages. Unlike those at traditional academy programs, academic center students will not have to wear uniforms.

The University High School for college-bound pupils and the Center for Visual and Performing Arts, also part of the system of magnet schools, will open at Suitland High School.

During the summer, Suitland and Pullen underwent significant changes to prepare for their new programs.

Pullen, for example, must accommodate 650 children in kindergarten through eighth grade after years as a middle school for grades seven through nine.

Classrooms have been coverted to studios and a new staff has been hired.

The magnet programs, now numbering 12, have been overwhelmingly popular since introduced in 1985. This year, even with 2,500 new slots in the magnet programs, all filled, more than 2,054 pupils are on the waiting lists. School officials say those students have little chance of getting into a magnet school this year.

Another program, not linked with the magnet program but expected to be as popular, is being scaled back because of lack of interest.

Last spring, 15 schools were chosen to have a before- and after-school program. However, school officials decided last week to limit it to nine elementary schools: Calverton, Carole Highlands, Hillcrest Heights, Hyattsville, Kenilworth, Marlton, Potomac Landing, Ridgecrest and University Park.

"At this point, we just don't have the money to underwrite it without a real show of parental support through preregistration," explained school spokesman Brian J. Porter.

Each program needs 60 to 75 regular participants to be self-sufficient. At those schools where the $35-a-week program was canceled, not enough parents signed up during a spring preregistration, Porter said.

At Barnaby Manor, Bradbury Heights and Capitol Heights elementary schools, teachers are preparing for a new crop of pupils younger than their regular students.

The Extended Elementary Education Program will bring 4-year-olds into the classrooms at those schools as well as at Chillum, Glassmanor, John Bayne, and Valley View elementary schools. Capitol Heights and Langley Park/McCormick elementary schools also are opening the prekindergarten classes, part of a state-financed program.

The half-day classes, which include morning and afternoon sessions, are designed to give youngsters a head start by working on reading and vocabulary.

"It's an achievement-oriented program. They're looking at raising performance levels . . . by starting them a year early," said Judy Hoyer, coordinator of early childhood programs.

The classes, which start Sept. 21, are open to children who live in the attendance boundaries of those schools.

Also this year, Morningside Elementary will open its doors as the county's 15th Milliken, or compensatory, school, with special programs for its predominantly black student body.

Elsewhere, Frederick Douglass Senior High School students will attend classes at the old Belair Junior High School while their school undergoes a $9 million face lift. The work is expected to take 18 months.

Things will not be so dramatic for students at Kettering Elementary School, which will open with tradition.

"At 8:05 the teachers will go out to the corridors and call the children's names that have been assigned to their classrooms," Principal Marr explained recently. "In 8 to 10 minutes, we can get them all in their classes."

And for the rest of the year?

Her answer has been repeated throughout the educational system.

"We look forward," she said, "with a lot of enthusiasm."