Tabatha McElligott did not know she was suffering from an undiagnosed heart condition which could have killed her at any minute.
When was was caught by a member of staff trying to sneak out of Westcliff High School for Girls during lesson time, she collapsed with shock at the prospect of receiving her first ever detention.
She and a pal were reprimanded by a teacher as they approached the school gates, but the ensuing panic caused Tabatha to collapse.
She told reporters: "When we got to the gates, I could hear my teacher calling my name, and my heart started hammering. She began to tell us off, but my heart kept beating harder and harder, and her voice sounded really distant. I felt my legs turning to jelly, and then everything went black. I now know my heart was a ticking time-bomb waiting to go off."
An emergency first aider arrived at the scene in three minutes and managed to restart Tabatha's heart.
The teenager was rushed to hospital where medics eventually diagnosed a rare condition that causes the body to pump blood around the heart the wrong way.
Tabatha, now 19, of Leigh-on-Sea, Essex, made a full recovery after surgery. She's extremely lucky as just 10 per cent of sufferers survive beyond their first birthday.
She has since trained to become a first aider herself - and her time off sick meant she never did get that detention!