Two year 11 pupils, Reagan Booth and Abby Western, were sent home from Rednock School in Dursley, Gloucestershire, last week because their hair was deemed "too blonde".
Reagan, a natural blonde who has had extra highlights for three years, has refused to change her locks, arguing: "No amount of hair dye affects a person's ability in school."
However, she fears her decision may lead to expulsion just months before she is due to sit her GCSEs.
School rules state that no pupil may dye their hair an unnatural colour.
Reagan's father, Martin, fully supports her decision, claiming: "Her hair looks a very natural blonde."
David Alexander, the head, has said the girls are not facing expulsion and were sent home simply to dye their hair. He accepts that a meeting with parents to clarify the rules will be necessary.