The school had to be evacuated on Tuesday afternoon because of a pungent odour in two classrooms which left many children complaining of dizziness, headaches and shortness of breath.
Police spokesman Johann Baumschlager said that it was still a mystery as to why the children fell ill, and that the strange smell seemed to have evaporated quickly.
The fire department said that someone may have intentionally taken a gassy substance into the classrooms as a prank, and that the substance was not natural gas.
A team of firefighters was called to the school at around 3pm on Tuesday to investigate a gas smell but their monitors did not discover any hazardous substances.
They spent five hours at the school and examined the entire school complex, as well as the ventilation system.
A total of 300 students and teachers were evacuated from the premises, which includes a secondary school and a business administration college.
Twenty-six students were given first aid on the scene and a further 38 were taken to hospitals in Tulln, Krems, St. Pölten and Vienna.
Firefighters examined the school early on Wednesday morning and said that there was no evidence of dangerous gases and that pupils could return to school. Some students decided to stay at home on Wednesday.