Doctors said the infection could have been caused by salads made from raw vegetables, which were served at the school canteen.
Meanwhile, Russia's consumer rights regulator, Rospotrebnadzor, said Monday that another possibility could be rodents at the warehouse where the vegetables were stored.
No pest control measures have been taken by school authorities to eliminate the rodent population.
The consumer watchdog added that the school headmaster would be held responsible for any breaches in sanitary standards.
A quarantine was announced soon after the outbreak started and all 539 pupils are being examined by medical staff.