An 18-year-old high school senior apparently distraught over personal problems killed himself yesterday morning by riding his motorcycle directly into the brick wall of his Fairfax County school.

About 9 a.m., as fellow students sat in class inside, Jerry Edward Minnick, of Franconia, positioned his motorcycle at the end of the track at Hayfield Secondary School, took off his helmet and then accelerated one-quarter mile into the wall of the boys' locker room, police said.

Minnick died instantly. A suicide note was found with his red helmet at the end of the track.

Police spokesman Michael Proffitt said Minnick had been upset about "personal problems," but declined to elaborate. Proffitt said he did not know whether Minnick was under the influence of drugs or alcohol, but added that toxicology tests will be conducted, as is routinely done.

According to his brother, ex-girlfriend, friends and teachers, Minnick came from a troubled family background: He never knew his father, his natural mother died in 1975, and he became estranged from his adoptive parents more than a year ago and was placed in foster care. In recent months, he had become a father and been involved in two traffic accidents, friends said.

His brother, John, 19, said Jerry was generally happy despite his troubles and gave no clue of his intentions when the two spent time together last weekend.

The note mentioned his antipathy toward his adoptive parents and "just briefly said, 'This is what I want,' " John Minnick said last night.

Joe Pasquariello, Jerry Minnick's social worker, said he could not discuss the case in detail, but said that no single factor leads to suicide.

"In cases like this, the motive is real elusive," he said. "You can't always find a motive, because there's more than one. Realistically, it's a combination of things."

Minnick, who lived with his foster parent, Ed Seward, in the 6000 block of Cromwell Place for the past year, attended Hayfield through eighth grade, then moved and attended Edison High School before returning to Hayfield in the middle of his junior year, his brother said.

Classmates and teachers described Minnick as a popular, friendly student who recently went to Ocean City for spring vacation and loved his motorcycle, a 1985 500-cc Honda Interceptor. "That was like his baby," Louis Marcil, a 17-year-old sophomore, said of the motorcycle.

Outside the school building, located on Telegraph Road five miles south of the Capital Beltway, more than 100 students stood and sat in silence as police investigated the scene. Friends cried, clutched each other's arms, embraced and, in a few instances, collapsed to the ground.

Dozens of students remained there for hours, long after police had left, even though there was nothing more to see, just staring at the wall Minnick's motorcycle hit. After school workers spread sand over the grass in front of the wall, about a half-dozen friends approached, dropped to their knees and stared at the crack in the brick wall.

One smoothed out the sand in front of the crack, while others fingered the crevice.

"I never got to say goodbye," one girl said while crying in her friends' arms.

By last night, a wooden cross had been placed in front of the wall along with flowers and two helium balloons that said "Thinking of You" and "You'll Be Missed."

Social workers rushed to the school and set up in the home economics lounge and career center to talk with distressed students.

A number of students and teachers said that when Principal J. Victor Lutz announced over the loudspeaker that someone on a motorcycle had been in an accident, they knew who it was because Minnick was so associated with his bike.

John Minnick arrived at the school after 11 a.m. and asked to see where his brother died. A police officer suggested instead that he go with him inside the school; the brother hesitated momentarily, but then let himself be led inside.

Jerry Minnick had a 5-month-old son, Christopher Simms, though he was no longer romantically involved with the mother, and was working at a service station in the months before his death, his brother said. Minnick had had two accidents in recent months, wrecking his foster parent's car, but emerging almost unscathed, friends said.

"This was the only real home he had," said one teacher who knew him. "He had this look of vulnerability -- you just wanted to reach out and hug him. It was like, 'Love me.' "

Pam Simms, the ex-girlfriend and Christopher's mother, spent most of the day crying.

"I just don't understand," she said between sobs. "He was a sweetheart. He had his moments, but he was a good guy."

Hayfield, which serves more than 2,500 seventh- through 12th-graders, has seen its share of student deaths in recent years.

Last year at this time, another senior was involved in a traffic accident the night before the prom and remained in a coma for days before dying. In spring 1986, three students died in just 18 days -- two in auto accidents and one from a health problem.