Police are hunting arsonists who set fire to a secondary school classroom in Scunthorpe, damaging GCSE coursework.
The attack on St Bede's RC Comprehensive School, in Collum Avenue, Scunthorpe, happened on Sunday.
A spokesman for Humberside Fire and Rescue service said it took about 20 minutes for firefighters to put out the blaze in a first floor art classroom.
Some GCSE artwork, due to go before examiners in about two weeks' time, suffered smoke damage in the blaze.
The spokesman said: "The perpetrators of this crime have carried out wanton vandalism but haven't thought about the effect it would have on the school and its young people."
The school's headteacher, Michelle Travers, stressed the exam board had been contacted and the work could still be assessed.
The attack came just two days after vandals damaged a wall of the school building.
The school head, Michelle Travers, said the school remained open but the art classroom was closed.
She said: "It is distressing, but fortunately we managed to keep the children's work."