At a secondary school in Essen, students drank an unknown liquid from a bottle on Friday afternoon and then had massive health problems.
A fire department spokesman said five students between the ages of 12 and 14 were taken to hospitals.
Four were seriously injured, two of whom fell unconscious after drinking.
Three other children had complained about problems even before the fire brigade was deployed and had been picked up by their parents.
The fire brigade secured the 1.5-liter PET bottle and took the first samples, which have so far yielded no results.
The bottle was therefore taken to the analytical task force of the fire brigade in Dortmund for further examinations with a fire engine and blue lights.
The bottle was apparently in a wastebasket in the schoolyard, the spokesman said.
It was initially unclear whether the students discovered the bottle there and drank it out of curiosity or whether someone offered them the bottle - a classmate or someone from outside the school.
The school had already closed classes, the spokesman said.
The liquid in the bottle is clear.
It cannot be judged right away whether it is poisonous or corrosive.
According to the fire brigade spokesman, the rescue workers were alerted twice: once around 12:00 p.m. when a child complained of nausea after physical education class and was taken to the hospital.
It is unclear whether this child also drank from the bottle.
A good hour later there was another alarm: four other children showed severe symptoms, two of them were unconscious.
The police are investigating for dangerous bodily harm against unknown persons, said a spokesman.
Children in the hospital would also be interviewed, as far as they were able to do so.
In addition, of course, the bottle will be examined for traces as soon as the fire brigade has finished determining the liquid.