The girl, who did not wish to be named, was told she was not allowed to attend the end of year celebration organised by her school, Outwood Academy Portland.
The A-grade student had even paid her deposit and bought her dress before she was told of the news two weeks before the prom was set to take place.
But the girl’s gradmother Sylvia Spencer, aged 67, said she was “elated” after the school’s u-turn just one day before the prom.
“We are over the moon because she did not deserve to miss out after what she has gone through. It is a nice surprise, but an unexpected one.”
Sylvia, of North Field Drive, Woodsetts, told the Guardian that her granddaughter has suffered from depression and anxiety in the last two years and was referred to and under the care of CAMHS, Child and Adolescent Mental Health Services.
Despite her problems, the youngster has already achieved a number of GCSE’s after taking some of her exams early in Year Nine and Year 10. The 16-year-old has also taken 17 exams this year and is predicted at least seven A grades.
The teen will be attending the Post-16 Centre next term.
“She is a high achieving young lady who is very talented,” Sylvia said. “She was very upset about I think she had been wrongly done to.
“The school have agreed that she is an outstanding pupil.”
The Guardian contacted Outwood Academy Portland but the school was unavailable to comment.
The prom took place at Aston Hall this week on Wednesday night.