A 15-year-old schoolgirl was yesterday sentenced to six months in jail after accidentally setting fire to a charity centre.

The girl had no previous convictions and was aged 14 at the time of the offence. At Minshull Street Crown Court in Manchester, Judge Adrian Smith also sentenced two boys, aged 14 and 15, to six months in jail for their part in the fire.

The blaze began after the group set fire to a bin outside a Children's Society centre last November. Three other youngsters – two aged 13 and a 14-year-old – were given two-year supervision orders for their part in the fire last Friday.

Derek Jones of the the Childrens' Society said the charity was "totally opposed" to jailing the girl. There was no question, he said, that the fire, which caused £30,000 worth of damage to the centre in Partington, Greater Manchester, had been anything other than an accident.

Mr Jones said: "Prisons are inappropriate and dangerous places for young people which put them at risk of bullying, abuse and violence."

The National Association for the Care and Resettlement of Offenders was also critical of the sentence. Policy director Paul Cavadino said: "Most children do irresponsible things and these irresponsible actions can have serious consequences But locking up children whose main crime is stupidity is hardly the best solution."

The National Association for Ex-Offenders, warned that the girl would be at risk of bullying and suicide as she mixed with hardened criminals aged up to 21 on the juvenile wing of Styal prison in Cheshire.

Chief executive Mark Leech said the organisation had received allegations that older inmates were targeting youngsters for sex at Styal.

Mr Leech said: "We should not be putting children in prison... it turns the young offenders of today into the old lags of tomorrow."