High school students joined co-ordinated university union protests in every major city and several smaller locations on Thursday.
The rallies, which Senator Hanson condemned on social media hours before they were held, began shortly after lunchtime.
In Melbourne, a crowd of about 1300 students and supporters gathered outside the State Library of Victoria before marching a loop through the city's centre.
Chants of "no hate, no fear, Hanson is not welcome here" and "we will fight and we will win, One Nation in the bin" filled the air well before the rally started.
High school students, some dressed in uniform, crowded the library forecourt, with organisers and police keeping them off the Swanston Street tram line.
Some had travelled far to take part, including 17-year-old Ballarat student Eliza Wynn who barracked for students at her school to participate.
"I saw the Instagram post (advertising the strike) and I was like, 'Oh my gosh, my school has to do that','" she told AAP.
Eliza said she had been politically-minded for years and her opposition to Senator Hanson and the One Nation party was firm.
"She promotes herself as a person who's there for the working class of Australia," she said.
"She's not."
Long Pham, a 16-year-old student from a local high school, said the rally was more a show of unity than political division.
"We want Australia to be inclusive," Long said.
"I believe everyone should be able to express their culture and shouldn't have that right be taken away."
After speeches, including one from former Victorian Greens leader Samantha Ratnam, a march shut down stretches of Swanston, Bourke, Elizabeth and La Trobe streets to traffic.
A second student strike is being planned for September.
Before the strikes, Senator Hanson said students should instead be protesting the cost-of-living crisis and stagnant national wage growth.
"The Labor Greens government is selling out your future, and you'll inherit a weaker, poorer and less safe country," she wrote on social media on Wednesday.
"Whether you know it or not, One Nation and I are fighting for you."
One Nation's primary vote has ballooned in the past 12 months, sitting at 26 per cent in early-August according to the latest Roy Morgan poll.
Further calls for students not to sideline their studies on Thursday came from politicians including federal Education Minister Jason Clare and Victorian Premier Ben Carroll.
"If people want to protest on the weekend, then fill your boots, but I want you at school when school is on, learning," Mr Clare told reporters.
Melbourne high school student Jules Arpula said there was "nothing more important" for students to do on Thursday.
"We refuse to just adapt to the new 'normal' of Hansonism," she said.
Senator Hanson used her June National Press Club address to call for a monocultural Australia before appearing on a podcast hosted by British far-right activist and convicted criminal Tommy Robinson during a work trip to the UK in July.
Victorian One Nation MP Rickie-Lee Tyrrell said the suggestion her party was racist was false and accused the Socialist Party of misleading students.
"These people are telling these children false information. We are not racist," she told reporters at state parliament on Thursday.
"If you have a look at our followers, our members, our candidates, there is a diverse range of people."
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