Kasia Ber, 17, was halfway through the school break when a mobile phone alarm triggered an undiagnosed condition which caused a heart attack.

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Now her family is taking legal action against the school and health chiefs after her condition went undiagnosed.

Mum Diane, 44, said: "How could they know our child was ill and not tell the people who were in the best position to care for her? If we knew we could have done something."

Promising law school student Kasia took ill on December 14, 2005, during classes at Byron Sixth Form, in Peterlee, with palpitations and shortness of breath.

Staff took her to a GP, but she told them not to tell her parents because she did not want to worry them.

She returned to the doctor four days later to undergo an electocardiagram, but was sent away with no further treatment.

On December 28, she collapsed at home after suffering a heart attack triggered by a phone, but all efforts to revive her failed.

Tests later revealed she had the heart condition Long QT syndrome which had gone undiagnosed. Her parents also later discovered she had been to see her GP just days before she died.

School bosses at St Bede's Comprehensive School denied any wrongdoing because they followed a House of Lords ruling known as the Gillick Principle which means parents don't need to be informed about medical visits and health problems if the child does not want them to know.

An internal investigation found school staff had acted appropriately.

But now Diane and her husband, John, 46, have instructed Hartlepool solicitor Andy Jones at MSP Legal, who is set to launch a high court battle.

The Ber's, of Brier Avenue, Horden, have finally broken their silence on the issue after more than two years because their son Christopher, 16, was a pupil at the school.

Mr Ber, who works for Unipress in Washington, added: "These teachers just let her go on her Christmas break with no protection and no support.

"It was a surprise to us when she died, but these people knew she was unwell. To us she looked like a picture of health, she was perfect."

The couple have also been through a series of meetings and a tribunal at the school, whose staff have remained steadfast in their decision.

Paul McKenna, headteacher at St Bede's, said: "We are aware of the complaint made by Kasia's parents, which was thoroughly investigated at the time, not just by the school and our governing body, but also by Ofsted and the Department for Children, Schools and Families.

"On each occasion the finding was that the school's actions were entirely correct and in accordance with the legal ruling known as the Gillick Principal."

After Kasia's death her family faced further heartache as they too had to be screened for the condition – and Diane was told she too had Long QT syndrome by a specialist in London – which was never diagnosed
Diane, who thought she had epilepsy, went through surgery at Newcastle's Freeman Hospital to have a defibrillator fitted.

She said: "If I had been diagnosed Kasia's life could have been saved because she would have been screened for the condition."

A spokesperson for County Durham Primary Care Trust said: "We are unable to comment on individual cases.

"However, we can confirm that the PCT's medical director wrote to all GP practices in County Durham and Darlington in September 2007 to alert GPs and practice nurses to the association of Long QT syndrome and sudden death, and the importance of this being investigated as a matter of urgency and specialist cardiology advice being sought."