The girl had drunk six litres of water in the game, and was pronounced dead in her home town after being declared brain dead in Sweden following the incident.
The school children played the game on the Åland islands located between Sweden and Finland, with the rules being that the loser of each hand on poker had to down a whole glass of water.
The girl’s water intoxication was not noticed by teachers until they were making a head count of the students later in the evening.
“She sat on the toilet and vomited. She had drunk an enormous amount of water,” said Jukka Silvola, principal of the school, to the Aftonbladet newspaper, adding that the girl was “truly well-liked”.
The 12-year-old was immediately taken to a hospital in Åland capital of Mariehamn before being transferred to specialist ward of the Uppsala University Hospital the next morning.