The incident took place on Tuesday at Ban Hmong Kao Lang School after the students were given boxes of chicken on oily rice as part of a merit-making ceremony.
Seventeen of the students had to go to hospital. A 12-year-old girl was pronounced dead early on Friday morning.
Mae Fah Luang district chief Worayan Boonnarat said 12 students remained in hospital yesterday – two in a critical condition.
He said he had instructed officials to investigate and find preventative measures to reduce the risk of something like this happening again.