Medical staff of the Sawang Metta Foundation and provincial hospital treated 46 fifth-grade pupils at Sukha Naree School for diarrhoea and vomiting.
Several more students stayed home after suffering the same symptoms late Monday night.
More than 110 pupils in three fifth-grade classes at the same school attended the police Drug Abuse Resistance Education seminar at a Mueang Nakhon Ratchasima hotel.
When the seminar concluded, they were each given a box of khao mun kai, oily rice topped with steamed chicken.
Teacher Sri-aroon Buakhamthum ended up with the same stomach upset as her young charges.
Two of the students were admitted to hospital with severe dehydration.