"Today is the day that we prove that as young people we have a voice to make a change for our future," Sydney event organiser Alexia Giannesini told SBS News.
"We are the people who are going to be most affected by climate change so it’s up to us to have the biggest voice and emphasise to our politicians that it’s our future that’s at stake.”
Student emcees said because they weren't able to vote, they were striking to make their voices heard.
"If you care about us, and the millions of people living on this planet right now, then you need to work with us," they said.
The protestors have three main demands: stop the Adani coal mine, no new coal or gas and 100 per cent renewables by 2030.
Thirteen-year-old Alex Buttler and Ashleigh Murray, who attended the Sydney strike, said it was ridiculous that they had to protest in the first place.
"Adults and people in power should have the common sense to not let it happen in the first place," they said.
At a similar action in Melbourne, hundreds of protesters filled the CBD streets with placards and chanting.
Brisbane's students also turned out in force.
But it wasn't just the capital cities that jumped on board, with events planned for Orange, Tamworth, Bendigo, Mackay, Port Macquarie, Alice Springs and more.
At the Cairns event, where hundreds of people turned up for the action, strike co-organiser Piper Lily O'Connell, 16, said: "we have less than 12 years to act and we need to take action now".
"My generation has done the least to contribute to climate change. We're going to be the ones who will be living through the worst of it," she told ABC News.
In Victoria, thousands of school-aged children packed the steps of Melbourne's Old Treasury Building and a nearby intersection.
Lewis, eight, and his friends said they were happy to be at the protest.
"We're here to support all of us and to save our future," he told AAP.
The primary school boy carried a sign saying "please save our reefs".
His mate Ethan, 11, was excited by the protest.
"I'm happy to be here," he said.
Schoolgirl Isobel Gilbert, 13, said she was protesting against Adani, the Indian-based company with substantial mining plans in Queensland.
"I don't want our north coast to be full of oil," she said.
In regional Ballarat there were reports of hundreds of students gathered for a similar protest.
The country-wide strike comes months after students first skipped school for the Strike 4 Climate Action protests, which brought thousands of kids out onto Australian streets - sparking condemnation from Prime Minister Scott Morrison.
"What we want is more learning in schools and less activism in schools," Mr Morrison said at the time.
In the lead-up to Friday's strike, the Prime Minister echoed his earlier sentiments, calling for "more learning in schools" and "less activism".
Senior government minister Mathias Cormann added that students "should not be used by professional adult activists as part of a cynical political strategy."
"During the school time kids should be in school," he told Sky News.
Meanwhile, NSW Education Minister Rob Stokes said students who turned up at the protest would be breaking the law.
"The law is clear and always had been, kids are required to be at school on school days," he said on 2GB.
"Turn up to school. Don’t rob yourself of the opportunities to get a great quality education.”
But NSW Opposition leader Michael Daley has backed the protests, a week away from the NSW election, describing the protestors as "future leaders".
At the Sydney event, City of Sydney Lord Mayor Clover Moore spoke to the crowd and said she supported the strikers, as a politician and as a former teacher.
Also a teacher, Janneke Thurow told SBS she wanted to expose her daughter Anoushka, 8, to "understanding this is one way to have a voice."
"I think activism is so important for this generation coming through and I want to give her a taste of that."
Source: Jennifer Scherer.
More than 90 countries are participating in strikes around the world on March 15.
The global movement began after 16-year-old Swedish student Greta Thunberg decided to skip school every Friday to protest outside the Swedish Parliament for more effective measures against climate change.
On Thursday, the teenager was nominated for the Nobel Peace Prize by a group of Norweigan politicians.