Sarah Turner had already taken delivery of a £350 dress shipped in from America after a special shopping trip with her grandmother.
But among Seaham School of Technology's criteria to go to its end-of-year prom is an attendance rate of 90 per cent or more – Sarah has only attended 85.55 per cent of sessions.
The 16-year-old and her mother Ann appealed to teachers to reverse their decision and argued her good behaviour and other aspects of her school life should have made up for the mark.
Headteacher David Shield told the Echo that although the school would not comment on individual cases because it bases decisions on personal information, each is judged on its own merits.
Ann, a shopworker who lives with her family in Eastlea, Seaham, said Sarah had missed classes because she suffers from asthma and claims the school sent her home on a number of occasions.
Six days were also missed in October because of the holiday to California Sarah was taken on by her grandmother Margaret Wanless, 60, also of Eastlea, which is when they found the dress.
Ann, 37, who lives with partner Derek Turner, 40, daughter Laura, 12, and son James, four, said she went through the appropriate channels to inform the school Sarah would be taking the time to go on the "chance of a lifetime" break.
Sarah said: "I'm really upset I can't go.
"I'm not the brightest of people, but I do try hard and do as I'm told. Most of my friends think it's pathetic."
Ann added: "She's gone to school since she was five years old and this is supposed to mark her final days and they say she can't go."