Lincoln High School Principal Al Graziano said a student was overheard talking about suicide Monday night, and later told authorities that others were involved. Officers went to each student's home early Tuesday.
Many involved were freshmen and sophomores, five girls and four boys, police said. They all were friends of William Metzger Jr., a 15-year-old who hanged himself last week following a car crash that killed three of his friends on Sept. 23. The boy's funeral was Monday.
"There are some problems to be dealt with, obviously," said Sgt. Tony Steverson, police spokesman.
Five of the students were taken to hospitals "to get some counseling, some help." He said one student "purportedly had taken an unknown substance" but was fine after receiving treatment.
Graziano said he didn't know what the substance was but that the girl had told authorities she had taken a sleep aid because she couldn't sleep.
Lincoln High School, just south of downtown Des Moines, has 2,200 students.
Freshman Cierra Jones was among those contacted by police early Tuesday.
"They asked if I had signed a suicide pact saying I was going to kill myself for Billy," Jones, 14, told the Des Moines Register. "I had no idea what was going on. I didn't hear about this or anything until they just showed up at my house this morning. I know I didn't sign anything. It's just really confusing."
Angellita Jones said she's been keeping close watch on her daughter since the deaths, saying "She has just been very, very upset about this."
About 29,000 Americans killed themselves in 2000, the last year for which statistics are available from the Centers for Disease Control and Prevention. It was the 11th leading cause of death overall, and the third leading cause of death for people aged 10 to 24, and the second leading cause of death for people 25-34.
The U.S. suicide rate in 10.7 per 100,000 — far below the leading rate in the world, in Lithuania, where 42 people in every 100,000 take their own lives.