"Strike for climate". With that simple plea, 16-year-old Swede Greta Thunberg has inspired children worldwide to boycott classes under the anxious gaze of adults who don't quite know how to react.
Every Friday since August, Greta, as she is known to all, has staked out a spot in front of parliament in Stockholm, demanding that her government step up the fight against climate change.
In the last six months, tens of thousands of high school students -- in Sydney, Brussels, Berlin, The Hague, London and other cities -- have followed suit. Some have marched in the streets bearing banners: "Save Our Future," "Act Now!", "Climate Change is Real".
"Our house is on fire," the charismatic teen told the global ruling class gathered last month in Davos, Switzerland.
"I don't want you to be hopeful, I want you to panic," she said with icy calm. "And then I want you to act."
"Her words penetrated people's hearts because they were spoken with such integrity," said Karen O'Brien, a professor of sociology at the University of Oslo working on climate change issues. "Sometimes it takes one person with courage to spark a fire that is just waiting to burn," she told AFP. "Thunberg articulated very clearly and courageously what the problem is and what needs to be done about it.
"Children have been listening, watching, and waiting -- they expected more from us and we have failed," O'Brien added, noting that they will likely be the first generation to feel the full impact of global warming.
An international protest movement led by young teens is "without precedent," said Sylvain Wagnon, history of education specialist at the University of Montpellier, in southern France.
"Their youth and in-your-face attitude are unsettling to adults, some of whom clearly are tempted to say 'you are just children, time to go home now'," he told AFP.
Upon learning that the education minister of New South Wales in Australia, Rob Stokes, had threatened to punish students and teachers for missing classes, Greta delivered a stinging riposte by Twitter.
"OK. We hear you. And we don't care," she wrote. "Your statement belongs in a museum."
Greta, who was to be in Brussels on Thursday and Paris on Friday, has called for a large-scale classroom strike on March 15.
There was pushback in Germany as well, where upward of 15,000 students have boycotted school during the last few Fridays in dozens of cities.
And during the first large-scale action in Britain last week, Prime Minister Theresa May's office complained that thousands of students walking out of class "increases teachers' work loads and wastes lesson time".
Once again, Greta fired back.
"British PM says that the children on school strike are 'wasting lesson time'," she tweeted. "That may well be the case. But then again, political leaders have wasted 30 years of inaction. And that is slightly worse."
- 'Follow their lead' -
In a video clip linked to her Twitter account, she added: "Some people thing we should be in school instead. But why should we be studying for a future that soon will be no more and when no one is doing anything whatsoever to save that future?"
"We are going to keep striking until our voices are heard and acted upon!", said 13-year-old Holly Gilligrand, one of the faces of the movement in Scotland.
Greta's army of schoolchildren has received ringing endorsement from many mainstream environmental NGOs and scientists, who have struggled for years to get the same messages across to a wider public.
"This is something new, something good -- I am absolutely convinced that we need a strong citizen-based movement," said Wolfgang Cramer, director of the Mediterranean Institute for Biodiversity and Ecology in Aix-en-Provence and a lead author on Intergovernmental Panel for Climate Change reports.
On Facebook, Cramer came to Greta's defence against social network trolls mocking the young Swede's Asperger's or suggesting that she had been manipulated by her adult entourage.
"It's shocking that some people believe that we can get rid of this kind of movement simply by making fun of its standard bearers," he said by phone.
Climate activists hope the movement will pressure governments, whose national voluntary carbon-cutting commitments do not come close to meeting the Paris climate treaty target of capping global warming at two degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial levels.
"I think it's remarkably hopeful -- but that hope depends on everyone else following the lead of these young people," Bill McKibben, co-founder of 350.org told AFP by email.
"It's not enough to stand back in admiration, because young people don't have the formal power to make change happen. It's up to the rest of us to back them up, and fast."