One out of every three eighth- and 10th-graders surveyed in 20 states said they have seriously considered killing themselves, and many have poor health habits that include drinking, smoking and ignorance of safe sex.

Thirty-four percent of the 11,000 students - 25 percent of the boys and 42 percent of the girls - reported that they had thought seriously about ending their lives, and 15 percent had gone so far as to attempt potentially fatal injuries, the national Centers for Disease Control reported Thursday.Dr. Lloyd Kolbe, director of the CDC's Division of Adolescent and School Health, said the suicide figures were surprising. "We just didn't think it would be that high," he said.

He said it was the first time the suicide question had been asked in the National Adolescent Student Health Survey, so researchers lacked a bench mark for comparison.

"Many schools in the nation are teaching more about suicide, about how to identify friends who may be suicidal and many school faculty are being trained in how to deal with suicides should they occur," Kolbe noted.

He thought the findings added urgency to such activities. "I think what it means is that we need to begin exploring this a little more and looking at what we can do."

The 1987 survey addressed questions about alcohol, drugs, violence, suicide and sex to eighth- and 10th-grade students in randomly selected classrooms chosen from a national sample of 217 schools in 20 states.

In the eighth grade, 51 percent had tried smoking and 77 percent had tried alcohol; by 10th grade, the numbers rose to 63 percent for smoking and 89 percent for liquor.

Overall, 22 percent reported smoking in the previous month and about 32 percent reporting having five or more drinks at one time in the previous two weeks.

"Drinking and drug use . . . contributes very substantially to homicide and suicide," Kolbe said.