Twelve students and the teacher were admitted to St. Cloud Hospital to be monitored overnight, hospital spokeswoman Jeanine Nistler said. None was seriously ill, she said.
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.
The cause was being investigated.
Nearly 20 students were in gym class when they saw bubbles coming out of the pool drain, Superintendent Scott Staska said. The water supply had been shut off for routine work in "back in the chemical room, actually," he said, and had just been turned back on.
The bubbles might just have been from air in the pipes, he said.
But the students began feeling sick shortly afterward.
"Quite frankly, we're at a loss to explain exactly what happened," Staska said.
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, although the pool may remain closed for a few days during the investigation.
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 4½ 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.
