Thirty-six students were injured by a chlorine gas leak in the pool area of a high school, officials said.
One adult also was taken to a hospital from Rocori High School, Mayor Doug Schmitz said. At least one other person was injured.
Six students and the adult will be kept overnight at St. Cloud Hospital for further treatment, authorities said. None of the patients was critically ill, said Thomas Schrup, the acting director of the hospital's emergency response team.
The leak was discovered before 2 p.m., and the building was evacuated, said Stearns County Chief Sheriff's Deputy Bruce Bechtold. The leak was contained within an hour, he said.
Nearly 20 students were in gym class when they saw bubbles coming out of the pool drain, Superintendent Scott Staska said. The students began feeling sick shortly afterward.
The gym class included ninth- and 10th-graders, some of whom were in the pool, Schmitz said. School was expected to open as scheduled Thursday.
The cause was being investigated.
St. Cloud Hospital set up a decontamination tent outside the emergency room for incoming patients, spokeswoman Chris Nelson said.
Chlorine gas can damage the eyes, skin, throat or lungs, according to the Centers for Disease Control and Prevention. Long-term health problems are unlikely.
"We've been through tough experiences here before," Staska said.
Rocori High was the site of a deadly shooting 4 1/2 years ago. John Jason McLaughlin was 15 when he took a gun to school on Sept. 24, 2003. He is serving a life sentence for killing Aaron Rollins, 17, and Seth Bartell, 14.