Two girls suffered burns after the roof of the main hall at Newent Community School caught fire during a performance of the musical Bugsy Malone.
Fire and ambulance crews ans-wered a 999 call on Saturday evening, after a pyrotechnic device, designed to shower glitter over the 100-strong cast, caused a small blaze on the ceiling.
Headteacher Delia Paveling ordered the evacuation of the packed hall and an extinguisher was used by a member of staff to put out the flames, before fire crews arrived.
Mrs Paveling said: "Within minutes the hall was clear and students helped the audience to the car park.
"Unfortunately, two girls suffered minor burns on their shoulders and arms from falling, melted wire. Both were treated by paramedics."
Both girls were well enough to attend school on Monday.
The device that caused the blaze had successfully released a glitter bomb over the cast on 11 occasions, during rehearsals and performances.
On Saturday, the electrical device released the glitter as planned, but on this occasion it also caused a fire on the ceiling.
Mrs Paveling continued: "I am tremendously proud of the way everyone responded. The students were really outstanding, showing great presence of mind and concern for the welfare of others.
"After such a wonderful show, it is pity not to focus 100 per cent on the display of talent from our young people. The performances were outstanding from everyone, on stage, in the house and behind the scenes.
"It was already an experience never to be forgotten."
She added that procedures would be reviewed in the light of the incident.
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