When the actor Morgan Freeman was growing up in Greenwood, Mississippi, there was only one movie theatre in the town.

Black people lived – and mostly still live – on the wrong side of the tracks. Every Saturday, little eight-year-old Morgan would troop up to the balcony with the other black kids in the town and dream about becoming a movie star. White children, meanwhile, took the seats on the main level.

Skip forward 40 years to the nearby town of Charleston where attitudes of the slave-owning past are proving even more difficult to uproot. The cotton fields have been replaced by lucrative catfish ponds and in town blacks and whites sometimes mingle in bars, shops and fast-food restaurants.

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Look a little closer and the ghost of Charleston's past still haunt the town. Only since 1970 have black and white students attended the town's one public high school, Charleston High. Two years ago, a black schoolboy tried to get into the segregated all-white high-school prom to see his friend. He was told he had to leave and then escorted out the door.

"But there isn't much tension," said Chasidy Buckley, who graduated from high school this year, "It depends on where you go. Some people are OK, some of the stores are owned by blacks, and some are by whites. Some of the stores that are owned by whites aren't – you know, you don't feel very comfortable in them sometimes but sometimes you do. But there is a black restaurant owned by black people and a lot of whites eat there."

Until relatively recently, the Mississippi delta region was the epicentre of the worst excesses of racism in America. A hotbed of Ku Klux Klan activity, it was a place of countless unrecorded lynchings. Just down the road from Greenwood, in a tiny hamlet named Money, one of the worst episodes of racial violence occurred in 1955. Fourteen-year-old Emmett Till, on summer holidays from Chicago, was beaten, shot and had his tongue torn out – for allegedly wolf-whistling or saying "bye, baby" to a white shopkeeper. His body was dumped in a river with barbed wire and a metal part of a cotton gin tied around his neck.

While the overt racial violence and the fears of the 1950s are but a dim memory today, beneath the surface, racial divisions remain. Greenwood has a black mayor and a black police chief. In the old days, white people rarely crossed to the black side of town. These days Greenwood hustles to attract tourists to its Delta Blues Trail. They stay overnight in sharecropper shacks and visit the grave of the blues great Robert Johnson, leaving flowers and bourbon in tribute.

But other places like Charleston, which fought tooth and nail against the reforms of the civil rights era, have proved more resistant to change. Despite the rulings of the US Supreme Court dating back to 1954, white residents saw it as an outrage that black students and white students would have to sit side by side in class. Only since 1970 – after a 16-year extension against integration on the grounds of economic hardship – have white and black students shared the corridors, classrooms and playing fields of the town's public high school, Charleston High.

And like many parts of America the ghosts of a segregated past still haunt the town. Many white residents just packed up and left. Today the town of just over 2,000 people is predominantly black, and integration has not come easy to the white families who stayed behind.

As long as anyone in Charleston can remember, high-school dances, including the annual rite of passage, the prom, have been segregated. Two entirely separate committees – one white, one black – made up mostly of parents, planned their own proms. One of the white parents reportedly told the (white) committee meeting: "I'm not going to have any of those niggers rubbing up against my daughter ..."

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It was only this April, 10 years after an upset Mr Freeman first offered to pay for an integrated dance, that the walls of separation came tumbling down and Charleston held the first integrated prom in its history.

Part of the credit must go to a Canadian film-maker named Paul Saltzman, one of the many who travelled to Mississippi in the 1960s to take part in the civil rights struggle. Last year, after discussions with Mr Freeman, the two men approached the school with another offer to pay for the prom, provided it was integrated.

This was happening as racial tensions were boiling over in the Louisiana town of Jena when black students were told they could not sit in the shade of a tree where white children gathered during the school break. After an assault, the authorities cracked down on black pupils, who received shockingly long jail sentences and before long the Jena Six had become a national cause.

Back in Charleston, the original school board, who rejected Mr Freeman's offer 10 years ago, had changed, as had the head of the school. The prospect of being filmed for a documentary, which had the blessing of a Hollywood star, also helped.

The director-producer, Mr Salzman, hopes that the feature-length documentary with Morgan Freeman, Prom Night in Mississippi, will be shown at next year's Sundance film festival.

Across the country, issues of racial integration have been grappled with for decades. Karyn Langhorne, a black novelist, recalls going to school in Long Island, New York, in the early 1980s where, by choice, black and white pupils ate in separate lunchrooms. When she then moved with her family to Georgia a fight erupted over the kind of music that would be played at the (integrated) high school prom.

"You would have thought it was the civil rights era all over again," she recalled. "White kids walked off the dancefloor whenever black music was being played. It was a battle royal."

After Mr Saltzman heard about the segregated prom in Charleston, he said: "I called up Morgan and I said, 'Is this true?' And he said, 'Yes, I made the offer 10 years ago,' and I said, 'Is the offer still good?' And he said, 'Yes'.

"And so we went to the school board and presented the idea of an integrated prom that Morgan would pay for. This time the school board accepted, and then Morgan presented the idea to the senior class, and we filmed him making that offer, and tracked the process for the next four months."

Mr Saltzman isn't quite sure what had changed in the 10 years since the first offer, except that the school board had changed, as had the superintendent and the principal. "I think the biggest reason is that the offer fell on new ears," he said. He then moved to Charleston for four months to get to know the community and win its trust. "I did a lot of civil rights work in the 1960s and there is no way I was going to barge in on the community with my cameras," he said.

For the prom itself, many of the pupils bought their dresses at the same clothes shop. The girls were practically colour-co-ordinated and most of the boys wore white. There was security outside, in case of trouble from outside groups opposed to the integrated prom, and, to everyone's relief, nothing happened.

"It was great to see white kids dancing with black kids," said Mr Saltzman. "White kids and black kids dancing with their same colour, but intermingling and having a ball. There were white kids dancing with the black kids, and singing every word of the hip-hop songs that they knew, and all the motions the same, so that, in a sense, the difference culturally dropped away in many ways."

For Ms Buckley the strain of racial segregation, however informal, is evident. "We have a 15-minute [lunch] break and all the whites are like in one area, except there are a few blacks and whites that hang together."

As she understands things, the reason for having segregated prom night is the parents. "That's the way that it's always been for them," she said. "But I mean, things have changed, so I didn't get why they kept on doing it. But they said, 'why change now? Let's just keep going'. That's the whole thing with our town. Everybody's afraid of change. It's just horrible." Like elsewhere in the US, racial tensions are "like a scab that nobody wants to disturb", said Mr Saltzman. Rarely are the issues discussed openly, although they are constantly talked about within the black and white communities. He discovered this after sitting down with the black chairman of the school board and the white school principal. "They both said, 'You know we've known each other for 27 years, but until now we have never talked about race.'"

The integrated prom night was deemed a huge success in Charleston. But it will take more than a prom and a documentary to change some attitudes. Some of the white parents still insisted that there would be a white prom. It was far smaller than last year's, but it went ahead anyway.