An Indiana teacher who used a much lauded bestseller, The Freedom Writers Diary, to try to inspire under-performing high-school students has been suspended from her job without pay for 18 months.

The effective book ban by the school authorities in Perry Township has outraged teachers and education reformers.

The Writers Diary, a series of true stories written by inner-city teenagers, was put together by a teacher, Erin Gruwell, and has been celebrated as a model for transforming young lives. It was made into a film with Hilary Swank last year.

Connie Heermann, a teacher for 27 years, sought permission to introduce the book to her students last autumn after attending a training workshop held by the Freedom Writers Foundation. "If you read the whole book you will see how these inner-city students grow and change and become articulate, compassionate, educated young people who want to do something good in their lives despite the environment in which they were raised," she told the Guardian. "I thought my students would very much relate to those kids."

Her head agreed and Heermann got written permission from nearly 150 parents, but the Perry Meridian high school board urged her to wait for its decision.

Teachers' union officials say that a single board member objected to swearing in the book. The school board member allegedly persuaded the other six officials to ban Heermann from teaching the book. It remains available in school libraries.

Heermann and the union say there was no explicit ban on the book when she handed it out to pupils on November 15. But later that day she received an email from the board advising her not to teach the book. "That was the pivotal moment of my life, when I saw how my students were taken with the book, how they loved it, and then I am told not to let them read it? I said no," she said.

After being threatened with dismissal, Heermann was eventually suspended. The union is deciding whether to take the case to court.

The school board denies book banning and accuses Heermann of insubordination. Barbara Thompson, the school board president, wrote in an email yesterday: "She knew she had defied her supervisors' direction in her work and that her defiance was 'insubordination' and 'neglect of duty'."