Around 90 people, most of them children, were affected by an irritant gas which caused respiratory problems at a secondary school near Constance on Wednesday.
Those affected at the school in Singen, just north of the German-Swiss border, suffered mild to moderate discomfort, the police said in the evening.
Three children and a teacher were taken to hospital after complaining of severe respiratory problems. A further 80 children who suffered minor injuries were treated in a hall, a spokeswoman said earlier.
Officials investigating the incident assume that an accident led to the release of the irritant gas. The source of the gas, however, was unclear.
Teachers evacuated the building after pupils suddenly complained of respiratory problems during the fourth lesson. They alerted the emergency services who cordoned off a large area around the building.
Some 600 students were evacuated from the building.
The police are investigating.
