Oskaloosa School officials here remained unsure Friday whether high school classes would resume on Monday.
"We anticipate we'll open on Monday, but we're not 100 percent sure," said Jim White, school superintendent.
Classes were closed Tuesday after administrators discovered that a student had brought 4 ounces of mercury to school and passed the substance around to other students.
On Friday, White said, state and federal environmental officials continued cleanup work at the high school.
After cleanup is completed, he said, workers will monitor the school for eight hours today.
An analysis of readings from monitoring equipment should be completed by Sunday afternoon or early evening, White said. The results will be used to determine whether the school will open Monday.
Phyllis Siefker, public information officer for the Kansas Department of Health and Environment, said officials had determined that no students suffered mercury poisoning.
She said tests performed on students who came in contact with the substance "did not indicate abnormal levels of mercury."
White said he remained unsure where the student obtained the mercury or why he brought it to school.
Handling mercury is not particularly dangerous, but long-term inhalation of the vapors can destroy the nervous system.
About 780 students in the district, including 265 in the high school, did not attend classes Wednesday because of the incident. The high school is housed in a complex that also contains the elementary and middle schools.
All students except those in high school returned to school Thursday.