The noxious smell sickened several children and prompted one class to take up quarters in a different section of the building for a portion of Tuesday, the day the problem started.
School officials theorize the foul smell plaguing Pioneer might be the result of a plugged sewer line coupled with stagnant air.
"It's objectionable," said Bob Wolpert, director of facilities and operations for Olympia School District. He likened the odor to the smell of rotten eggs.
After first arising early in the week, the smell recurred sporadically throughout the week, prompting officials from the school district and the Thurston County Health Department to investigate.
Principal Esther Baker sent a letter home with students Thursday reassuring parents the campus is not dangerous, though some students came down with headaches, dizziness and drowsiness.
"It drifts around a lot. It's a strange thing," said Baker, who reported that parents generally haven't been alarmed by the situation.
The smell seemed to enter the building through three separate vents that normally discharge air from around the restrooms and sinks, Wolpert said.
Instead of being carried away by the wind, a stagnant weather system kept the odoriferous air close to the building, allowing it to get pulled back inside when the heat pumps engaged.
As a stopgap measure, the three vents have been extended about 6 feet over the building to help move air farther away.
Two vents are over the school office and the other is over classrooms used for remedial study and for fifth grade.
Health officials and school maintenance staff have examined all the vents.
Wolpert said one theory is that a sewer line might be plugged either on campus or in a public line running near the property, which would account for a strong smell reported outside the school.
The district is cleaning out Pioneer's sewer lines, and has asked the city of Olympia to check its lines as well.
The problem came and went all week, but seemed to be dissipating, Wolpert said.
"Today, everything is clear," he said Friday.
Wendy Culverwell covers education for The Olympian. She can be reached at 360-754-4225 or wculverwe@olympia. gannett.com.