You remember the film, of course. Sidney Poitier tames an unruly bunch of Swinging Sixties East London teenagers and Lulu sings “To Sir With Love” to him at the end of term disco. Possibly you don’t remember the original book, the semi-autobiographical work of E R Braithwaite (who, aged 101, attended the Northampton press night all the way from his home in New York), about the erudite and suave West Indian immigrant, Ricky Braithwaite, and his experience of school teaching under an experimentally liberal headmaster.

Ayub Khan Din’s adaptation of the novel returns the story back to its late 1940s setting, a time of poverty and deprivation for many; a post-war era where the indigenous white population were learning how to live side-by-side with new black neighbours. Ricardo Braithwaite has had a successful war in the RAF where he faced no prejudice; but out on Civvy Street three years later, life is very different. Like his creator, Ricky wants to use his degree to get a good job in engineering, but despite his academic qualifications, no employer will give him a chance.

So he approaches Greenslade school to see if teaching might be an option; and within a few hours of observing the place finds himself hired and in charge of the terrifying senior class who will be leaving at the end of the year. Of course, he faces disruption, disobedience and distasteful behaviour from his students, who are the complete opposite of him in terms of education and demeanour. But egged on by his very forward-looking headmaster he finds other ways to gain their confidence, and progress is made. In the unlikely event that you don’t know how it all turns out, I’ll say no more.

If ever there was a play that shows how harmful and hurtful casual racism is, this is the one. There were many gasps of shock and amazement from the audience at some of the comments made by the world-weary cynical teacher Mr Weston, which you could take to be the attitude of the “old guard” as it were. There’s also the anti-Jewish comments made by one of the students, so close to the Second World War, which Braithwaite is quick to condemn. The effects of racial discrimination are painfully sharp, shown in the attitudes of both local working-class families and nice middle class people from Berkshire. It both disturbs and disappoints you. Because of the way it highlights and shames racism, Mrs Chrisparkle thinks the play should be compulsory viewing for all schoolchildren.

It’s not just about racial prejudice of course; it’s a very interesting look at a progressive educational system (not without criticism), the effects of poverty and the class structure, the nature of bullying, and about friendship and love. At two and a half hours, it’s quite a long play by today’s standards but it certainly doesn’t feel it. We were both totally engrossed by the story and the performances that it absolutely flew by. Mike Britton’s set makes good use of the traditional school blackboard, and effectively recreates the comfortless, hostile environment of the old Victorian school room.

At the heart of the story is Ricky, played by Ansu Kabia. He is perfect for the role, with his dignified bearing and crystal clear authoritative voice. It’s a superb performance from someone you can absolutely believe is that teacher, doing his best to rise above adversity to make a difference in the world, but who is only human too and reacts emotionally to injustice and personal slight. And what a trooper! After being knocked off his bike, breaking a finger and injuring his hand, he performs the play with his arm in a sling. The show must go on, and Mr Kabia’s devotion to duty is pretty goddam remarkable.

Matthew Kelly, who we last saw playing another “unusual” teacher in The History Boys, is the maverick headmaster Florian. Whereas Hector was a law to himself in the classroom and lacked support from the powers that be, Florian is the man with the real authority, which Mr Kelly portrays with quiet, confident reason. It’s a really well thought through performance, and his conversations with “Sir”, helping him to establish a rapport with the children, are beautifully judged and constructive.

Paul Kemp plays the irascible Weston, a pipe-smoking little bigot of a man who would really get under your skin if he were a colleague of yours. Mr Kemp gives him a brilliant weedy, whiny voice which helps emphasise his smallness of mind and vision. Weston gets his come-uppance, in a scene which Mr Kemp plays superbly well – you could hear a pin drop during his discomfited speech. The character is one of many who develop into different people at the end of the play from that which they were at the beginning. Nicola Reynolds is great as Clinty, the domestic science teacher – a good-hearted, no-nonsense character whom she really makes come alive. Unlike Weston, she’s the kind of character you really would like to have as a colleague. She has excellent comic timing in many funny scenes but also creates a lot of pathos where it’s needed. Peta Cornish is the other teacher Gillian, inexperienced at both teaching and at life, who falls for Ricky and very sweetly conveys the tentativeness and embarrassment of her emotions, and is simply lost when it all goes wrong.

There are some great performances from the young actors too. I was really impressed with Mykola Allen as Denham, the most disruptive of the class, whose power base starts to ebb away as Sir becomes more popular. A more sour-faced difficult kid you could never hope to meet, it’s a very intelligent study of someone set in their own ways who refuses to change even when it’s the obvious thing to do; it’s also a great portrayal of a bully who gets what he deserves. His final scene is played with superb controlled emotion. Harriet Ballard, too, gives a great performance as the boisterously difficult Monica, all pretend effrontery and a challenge to anyone’s teaching skills, and whose visible softening is very believable and heart-warming to watch. Kerron Darby gives a very touching performance as Seales, troubled by his own racial heritage; his reaction to bad news really tears at your heartstrings; and Heather Nicol gives an excellent performance as Pamela, the girl with a crush on Sir, making the best of all the situation’s potential for comedy and heartache. It’s a shame that the programme lists the rest of the cast as “ensemble” as they are distinctly different individuals with their own lines, characters and scenes. True, they did work together seamlessly as scene-shifters and dancers, but I think they should have been individually credited to their roles. I’m sure Sir would have thought each would deserve their own recognition.

Anyway, it’s an excellent production and a thoroughly enjoyable, funny and moving play. It’s the first in a new series of co-productions between the Royal and Derngate and Touring Consortium Theatre, of which we will see one a year for the next three years – sounds like something definitely to look forward to. After it leaves Northampton, it’s touring the country until the end of November. Go see it!