Three senior high school students in Shanghai were slightly burnt on May 16 during a chemistry experiment popular among students -- producing bombs made from chalk and chemicals, Youth Daily reported today.

"They said they were burnt when trying to mix red phosphorus with potassium chlorate," said a doctor, surnamed Lu, from Changhai Hospital, where the victims had been sent to.

The girls are in stable condition, Lu said. "But one of them has to receive further treatment as several finger bone joints were badly burnt."

The senior high school students of grade two at Fuxing Middle School and their teacher refused interviews. The story didn't say if the incident happened during classes.

The formula of "chalk bomb" can easily found on the Internet, said the report.

Glaring light, a big noise and white fog, the three basic effects of the blast, may happen when the "bomb" is used, it said.

A chemistry teacher, surnamed Chen, from Luwan Senior High School, said students are not allowed to do dangerous experiments in classes.

"Though the 'chalk bomb' produces clear blast effects, which is a good teaching example, few schools let their students try themselves," Chen said.