Free speech controversies are like solar flares.

They burn hot and bright. Right now, it is Angus that is feeling the heat. Last week, the Sunday Herald reported that one headteacher in Kirriemuir had pulled Black Watch off the Highers syllabus because it is "offensive". Parents are angry at the decision, and have demanded an explanation.

Freedom of expression does not just mean the freedom to write or say what you please, but also the freedom to read and to hear what you choose. The decision to remove Black Watch from the classroom curtails the right of the pupils to read and study one of Scotland's most culturally significant plays. Moreover, the essays that they have already written on the play will not be assessed.

It is entirely right that prominent figures in Scottish literature have written an open letter, urging the head to reverse her decision (in signing the letter they, too, are exercising their right to free speech). This decision may just affect one school, but that is enough to set a precedent. The free speech issues have been raised and must be debated before any more books are removed from shelves and school-bags.

It is particularly important that we challenge 'offence' as the justification for such decisions. If we do not, we run the risk that 'offence' becomes ingrained as a legitimate reason for censorship. We put a veto-power in the hands of whoever says they are upset. Offence, and its sibling, indecency, are the perennial free speech battleground in British society, and often it is literature over which we fight. Think of the fatwa issued against Salman Rushdie for The Satanic Verses; think of Mary Whitehouse's crusading legal actions against plays and poems that depicted homosexuality; think of Lady Chatterley's Lover, prosecuted for obscenity.

During the Chatterley trial, the prosecutor Mervyn Griffith-Jones was criticised for asking whether the book was something "you would wish your wife or servants to read". This paternalism is often at the heart of classroom censorship - the idea that the kids are too young to comprehend the subtleties of art. Scotland had this debate in the 1990s when Edwin Morgan's Stobhill sequence of poems, which depict rape and abortion, were the target of a campaign to have them banned from schools. Down in England, 'Education for Leisure', Carol Ann Duffy's chilling poem about a frustrated young man with a knife, was pulled from the GCSE textbooks after critics said it 'glorified' knife-crime.

The United States, where even the most parochial levels of government are highly politicised, has endured many battles over what books should be read by children. Since its publication in 1900, various public libraries and parents groups have sought to suppress The Wonderful Wizard of Oz, and in recent years the Harry Potter series has been attacked because it promotes witchcraft. Another book that is frequently a source of contention is Mark Twain's Adventures of Huckleberry Finn. It is often described as the first great novel of American literature, and yet it also carries 219 instances of the N word. The characters that use it are undoubtedly racist by modern standards, but the book itself-the story of an escaped slave -is far more humane than the people it describes.

In Black Watch, the contentious word is 'c**t' which the characters use routinely. C-bombs are dropped into conversation with far more regularity than the sound of actual bombs falling on the Basra military compound where the play is set. Sometimes, the word seems benign, as if the soldiers think it is synonymous with 'man' or 'person'. But this is not always the case, and often it is deployed as an insult. The c-word has a sexist history and meaning and there is no escape from that legacy.

Worse, the characters talk constantly about various sex acts with the women they have met, and use derogatory language about gay men. There is no denying that the characters are offensive. Perhaps they will corrupt the morals of our young people? Will the swearing instil negative values in those who read and watch the play?

In all these attempts to shield young eyes from bad words-whether its Huckleberry Finn, or Black Watch-there sits an implication that children cannot grasp the full meaning of the text. For primary school children, there might be some merit to that argument, but it is patronising when applied to teenagers studying for Highers. Last year, 16 and 17 year-olds in Scotland were asked to vote on the complex question of Scottish Independence. To suggest that these same citizens cannot be trusted to read about characters doing offensive things, is just bizarre.

Moreover, drawing a distinction between what a character says in a play, and the playwright's message, is surely the very essence of literature studies. In a classroom, the offensive words are not presented alone, but within a highly specific context that a teacher must explain. Indeed, I would suggest that a school is the best place to uncover that context. Those who say that the kids can always read it at home if they want are denying them the chance of a deeper understanding of the play and the issues it raises.

Could the play exist without the swearing? Would it make as much sense if the c-words were all replaced with something else ('clown', say). Why be needlessly offensive?

The trouble with that suggestion is that for many playwrights, the offensive aspects are not gratuitous but essential to their message. Another British play that has been de facto banned is Gurpreet Kaur Bhatt's Behzti, which features a rape and a murder in a Sikh gurdwara. Many people protested the play, and prominent community leaders accused Bhatti of being deliberately provocative. But as anyone who has seen the play can testify, the gurdwara setting is essential to the piece, because in a place of worship, the rapist can control the other characters. The shocking setting is therefore what makes the play work, and the equally disturbing themes are what makes it necessary. It is appalling that no theatre company has had the guts to re-stage Behzti, for fear of offending.

Just as the shocking setting is essential to Behzti, the offensive words are crucial to Black Watch. During the summer of 2006 I was lucky enough to work on the original NTS production as part of the video design team. In dozens of rehearsals, my ears were bombarded by the fruity language. Sat in the darkness of the Edinburgh Drill Hall, I had plenty of time to consider whether it was necessary.

By the opening night, I had reached my conclusion: the play would be rubbish without the bad language and sexist banter. Ironically, if the strong Fife vernacular of the characters were cleaned up, the message of the play would become incoherent.

There are several reasons for this. The first is a matter of authenticity. Gregory Burke's play is based on interviews with former soldiers. It intends to be a story of war told from their point of view. For this story to be told properly, it is essential that their real voices are heard, because it is through their words that we get a genuine sense of what these men are like. The soldiers we send off to kill and die in the Middle East are not swashbuckling toffs speaking with received pronunciation. They are working class, inarticulate and insecure boys with no prospects other than the army. And when these men speak, they swear. It is integral to their vernacular. To sanitize their words would be to silence them.

The play even makes a specific promise to the soldiers not to do this. Black Watch has meta-scenes where a playwright interviews the soldiers. At one point they ask him about his intentions: "Are you gonna use us?" He promises to report truthfully what they say.

By the way, the person responsible for the most inauthentic part of Black Watch was... me. On of my tasks on the show was to choose a 20 second porn clip for the characters to watch during one of the scenes. I chose something laughably soft-core in the hope of not offending too many people, but I'm certain it is not the sort of thing that the soldiers would actually have watched.

There is no doubt that Black Watch is a shocking play. If it is performed right, the audience should be made to feel deeply uncomfortable with what they are witnessing. But this shock does not come from the language, but through the story: that of a group of men lured into the army, sent to fight a war under false pretences, and broken by the experience... all while the regiment itself is being gutted by suits in Whitehall. It is a Scottish story, the first and vindicating production of the National Theatre of Scotland, that has been applauded by audiences around the world. To hear that the school children of Angus, a Black Watch county, are to be denied the chance to engage with this story on the grounds of 'good taste' is, to my mind offensive. Scottish history is bloody and difficult and its people are raw and rude. Our children should be taught to celebrate that fact.

Robert Sharp is Head of Campaigns at English PEN, the founding centre of the PEN International network of writers campaigning for the freedom to write and the freedom to read. From 2002-2007 he was a director of Edinburgh-based video design company 59 Productions