According to one report the girls had lit a fire to keep warm.

A dozen girls were found poisoned at the Duiziliang Middle School in Yulin city, said an official with the Yulin Education Bureau.

She said 11 girls were confirmed dead, with another being treated at a hospital.

The Xinhua News Agency said the girls died on Monday after lighting the fire to keep warm. The report said the 12th girl was in serious condition.

Carbon monoxide detectors aren't required in schools in China, although the Education Ministry last year asked schools which can afford it to install detectors if they burn coal for heating.

The Xinhua report did not give the ages of the girls or other details. It said the provincial vice governor was helping to investigate the case.