Tracey Meaning, the former head of spiritual and moral guidance at Minster College in Sheerness, Kent, took the action when she feared that one of her brightest students would fail to get into college.
Mrs Meaning, who now works as a police support officer in a community college in Kent, believed the girl was capable of achieving a B grade but that problems at home meant she would fail.
She told England's General Teaching Council this week that because of family difficulties the girl's attendance at the comprehensive had been sporadic.
Regarded by students and the community as a role model, Mrs Meaning entered a boy's coursework under the girl's name, without her knowledge or consent, in April or May 2002.
"My decision was completely misguided and came from a situation which was beyond anyone's control," she told the GTC. "I made a serious error of judgement to try to short-circuit the system based on my knowledge of her abilities."
She said the girl needed the grade to go to college but was unable to do so in the end because of her family problems.
Ruth Rundle, acting head of the college at the time, said that Mrs Meaning's actions had affected the comprehensive's reputation and that the exam board could have banned it or subjected it to extra regulation.
She said that Mrs Meaning, who qualified as a teacher in 1999, was inexperienced and overloaded with a number of new projects.
Mrs Rundle said: "As an RE teacher, many of the pupils thought of her as a behavioural touchstone."
The boy achieved a grade B in the RE GCSE, which was part coursework, part exam. The girl got a grade D.
Mrs Meaning's deceit was discovered only after the boy had searched a cupboard for his old projects.
She apologised for her actions and also admitted to the GTC hearing in Birmingham that she would not have told anyone about her behaviour if it had not been discovered.
Valerie Cox, chair of the GTC panel, said she had not only duped a colleague but also breached society's trust in teachers to administer exams fairly.