The patients were admitted Wednesday.
"The children have the same symptoms--vomiting, diarrhea and dehydration, while some are also in pain. It is now being assessed who of them can be treated at outpatient clinics or at home, and who has to remain in hospital," the Director of the Children's Hospital, Zoran Radojicic told B92.
The cause of the poisoning was staphylococcus bacteria and there will be no long-term effects of the incident, health officials said.
The food was likely contaminated during preparation, but the exact source has not been identified.
The Milean Pavolovic Barili school receives meals from a local restaurant, which has denied that they provided poisoned food.
The restaurant provides meals for 21 elementary schools and other students did not suffer any illnesses, the restaurant said.
Health officials, however, have prohibited the restaurant from providing meals until an investigation is complete, B92 reported.