Several students in a middle school in Shanghai have been taken ill after consuming water that had been tampered by two classmates, the Pudong district police announced on Thursday.

Following investigations, two 13-year-old girls were found to have sneaked into the classroom when their classmates were having a physical education class on Tuesday afternoon. The two girls then poured cleaning and bleaching agents into the water bottles of their classmates.

All students who felt unwell were sent to the hospital for treatment and were discharged on the same day.

Pudong district police said that classes have resumed and that it is collaborating with the education authority to conduct further investigations.