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The 17-year-old posted a photo on social media showing her wearing a backpack and a caption saying: "My gap year from school is over, and it feels so great to finally be back in school again!"
She's certainly achieved a lot in the last year.
Greta sailed from Europe to North America and back again, spoke at the UN, which went viral around the world, led a global climate strike in September that was one of the biggest collective actions in history, was invited to speak at World Economic Forum in Switzerland, and inspired millions of people around the world to think about the planet's health.
Despite this whirlwind of 12 months, she recently said very little has changed.
Thunberg said: "Effectively, we have lost another two crucial years to political inaction"
Writing in The Guardian alongside fellow climate activists Luisa Neubauer, Anuna de Wever and Adélaïde Charlier, the group wrote: "Looking back [over two years], a lot has happened.
"Many millions have taken to the streets... and on 28 November 2019, the European parliament declared a climate and environmental emergency.
"But over these last two years, the world has also emitted over 80bn tonnes of CO2. We have seen continuous natural disasters taking place across the globe. Many lives and livelihoods have been lost, and this is only the very beginning.
"When it comes to action, we are still in a state of denial. The gap between what we need to do and what's actually being done is widening by the minute.
"Effectively, we have lost another two crucial years to political inaction."
Thunberg's protest began in August 2018, in the run-up to Sweden's general election, when she decided to stop going into classes at her school in order to draw attention to the climate crisis.
That particular year saw Sweden hit by heatwaves and wildfires, and was the hottest summer in the Scandinavian country since records began 260 years ago.
While her solo campaign started small, it slowly gathered pace until millions marched around the world in September last year.
Even though Ms Thunberg is heading back to school, she isn't going to give up her mission to get more global action against climate change and hopefully protect future generations from suffering.
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