Students from Stoke Newington School in north east London had Thursday off because it was "progress review" day. But it might just as well have been Stop Kony day. That's all they wanted to talk about.

Six 14- to 15-year-olds gathered in Fergus Mason's house to discuss Kony 2012 – what the film meant, what the next stage of action was, how to stop the rebel leader. There was a sense of astonishment: that one man could be responsible for so much evil, that a film could have so much power, that they had the potential to change so much.

Few of them had heard of Joseph Kony, the leader of the Lord's Resistance Army, before the video Kony 2012 went viral on the internet. But now teenagers all over the world can tell you how his army, which has wreaked havoc for 26 years, rapes little girls and forces child soldiers to murder their parents.

But if this film tells us anything about the 21st century it is about the power of celebrity and social networks. By 7pm on Thursday, nearly 50 million people had watched it – mostly because their friends had shared it with them on Facebook or their heroes on Twitter had alerted them to it.

P. Diddy tweeted "Dear Joseph Kony, I'm Gonna help Make you FAMOUS!!!! We will stop YOU #StopKONY ! All 6,000,000 of my followers RT NOW!!! Pls!" "#KONY2012 Spread the word!!! PLEASE go to Invisiblechildren.com Even if its 10 minutes … Trust me, you NEED to know about this! #1LOVE"

Rihanna tweeted to her 14 million-plus followers, and Justin Bieber told his army of 18 million: "SO glad you're behind this! He MUST be stopped! THANK YOU for helping spread the word. POWER IS IN NUMBERS. #STOPKONY."

And so it came to pass; the word spread like wildfire.

Fergus said the first thing he was going to do was share it with as many people as possible. Owen Watts-Moore felt it was now the responsibility of children to educate their parents – to make them pay attention to an atrocity they turned their back on.

He loves the way the film is inverting the traditional notion of celebrity. "It's a clever idea – making Kony famous for his evil deeds."

On the Invisible Children website, the film's director Jason "Radical" Russell is described as "co-founder … as well as our grand story-teller and dreamer".

In Kony 2012, he tells us that Kony is the number one wanted man in the international criminal court and shows how his campaign has led to Barack Obama sending 100 US advisers (mainly soldiers) to hunt Kony.

"It was the first time in US history that the American government took action not because of self-defence but because people demanded it."

Occasionally, it seems that Russell is encouraging acts of vigilantism (a citizen's arrest on Kony?) but gradually it becomes clear that he is asking the world's children to help with publicity, money and care.

Lily Goldblatt was still emotional from seeing the film. "It was so moving. My BlackBerry was going crazy with people watching the video. Everybody was really upset, crying. It's such a powerful message."

What is that message? "Children younger than me are being turned into murderers by Kony."

It's funny, Lily said, a couple of days ago she didn't have a clue who Kony was. "Then everything started coming up Kony, Kony, Kony and I was at a loss to what it was."

But the more the youngsters think about the film, the more questions they have. After watching it, they researched Kony and Uganda on the internet, and came to the conclusion that the film was partial with its history and selective with its facts.

"It's good, and very well meaning," Fergus said,"but it's very simplistic. There are other sides to the story they don't talk about. The Ugandan troops are not exactly saints, and they do glorify them a bit."

The thing is, said his friend Dan McAteer, since Russell first went to Uganda to make a film about the LRA in 2006 the situation had changed – the LRA and Kony are thought to be something of a spent force.

"The film focuses solely on Kony and portrays him as an antichrist. You'd think Uganda is fine apart from Kony. But actually the government the film asks us to support is considered one of the most corrupt in the world. This is the same government that called for the killing of gay people to increase its popularity."

Perhaps, inevitably, criticism of Kony 2012 has come thick and fast over the past 24 hours; it has been suggested that Russell is playing god. Is it right that a white middle-class American filmmaker should ask us to save Africa from itself; is the interview with George Clooney really supporting this particular film?

Charlie Couve, 15, was worried about where the money raised by Invisible Children had gone. He pointed out that according to its own financial records, the organisation spent $8.9m (£5.6m) and only $2.8m actually went to help Uganda.

Dan agrees that the film overstates the case for the power of the individual and Facebook. "The guy in the video says this is now a Facebook world, and the big pyramid with money at the top and people at the bottom has been turned on its head. Well I don't really think it has, because this is the first major event I've seen our lot [his friends] involved with. And in the end they're still lobbying the same governments to do the same things they would have 20 years ago. The only difference is people hear about it quicker because of social networks."

But for all their scepticism, the teenagers in Stoke Newington think Kony 2012 has to be a good thing. "I didn't realise how sick one person could be and create such problems in a place that doesn't need any more," Ella Barry said. "It encouraged people to believe they can make a difference by speaking out."

In the film, Russell promises a global day of action on 20 April, "when we meet at sundown and blanket every street and every city till the sun comes up. The rest of the world will wake up to hundreds of thousands of people demanding justice in all corners."

He tells his supporters: "We are not just studying human history, we are shaping it" and urges youngsters to buy the bracelet and action kit and donate what they can. "We are the power," he says. "A bunch of Littles can make a big difference."

So what will the youngsters do next to bring Kony down? Most said they had already been on the internet to buy the bracelet and action kit, but came away a little disillusioned.

"Last time I looked it was $10 a bracelet," Charlie said, "and then $20 more for overseas delivery, which I think is just … well it doesn't cost $20 to send a package overseas."