Climate advocates celebrated Thursday after New York City's public school system announced it would excuse the absences of students who have a parent's permission to participate in the global climate strike on Friday, Sept. 20.

"Holy cow!" tweeted Bill McKibben, a co-founder 350.org. The environmental group is helping plan a week of action that will feature thousands of events around the world to coincide with a United Nations climate summit in New York City.

Another 350.org co-founder, Jamie Henn, also welcomed the NYC announcement on Twitter Thursday. He wrote, "holy smokes, this thing could get HUGE."

The NYC Public Schools account explained in a series of tweets that younger students will only be allowed to leave school for the climate strike if they are accompanied by a parent and promised to share guidance with the city's schools and promote class discussions about the climate crisis.

.@NYCschools will excuse absences of students participating in the #ClimateStrike on Friday 9/20. Students will need parental consent. Younger students can only leave school with a parent. https://t.co/hcBO1Cnb3m — NYC Public Schools (@NYCSchools) September 12, 2019 We will share guidance with schools, and encourage class discussions about the impact of climate change, and the importance of civic engagement. #ClimateStrike — NYC Public Schools (@NYCSchools) September 12, 2019

Alexandria Villaseñor, a 14-year-old NYC leader of Fridays for Future and the first American to hold a school strike for climate, noted in a statement that the New York City Department of Education's decision came "after many months of hard work and conversations between we activists and the city."

"They are finally treating the crisis like a crisis and this will make sure that September 20th is the largest mobilization that New York City has ever seen," said Villaseñor. "Now, I am calling on every city in the U.S. to follow New York City's lead, and support their students in striking for their futures. We need everyone in the streets next Friday!"

"It took me only 41 weeks to be excused from school for my #ClimateStrike," she added in a tweet. "Never, ever give up people."

Thank you NYC Councilmember @bradlander and the entire @NYCCouncil!

We're going to have the biggest strike New York City has ever seen! https://t.co/ppxYk9cpVr — Alexandria Villaseñor (@AlexandriaV2005) September 12, 2019

NYC's announcement also came just a day after Amnesty International secretary general Kumi Naidoo sent a personal plea to more than 30,000 schools worldwide urging administrators to allow students to join the climate strikes on Sept. 20 and 27, which will bookend climate campaigners' week of action, inspired by youth activists with the Fridays for Future movement.

Greta Thunberg, the 16-year-old Swede whose protests for bold climate action outside her country's parliament last year help sparked the global school strike movement, highlighted Naidoo's letter in a tweet Thursday.

The Secretary General of @amnesty International has written to over 30000 schools, asking them to allow students to take part in the climate strikes on September 20-27th!#FridaysForFuture #climatestrike #Schoolstrike4climate https://t.co/tkpyGMnOsi — Greta Thunberg (@GretaThunberg) September 12, 2019

Amensty's U.S. chapter shared a video on social media Thursday featuring 15-year-old Kallan Benson from Annapolis, Maryland, who explained that through youth-led movement, "we are calling on our leaders to listen to the scientists and listen to the science that is coming out about climate change," and to do more the addressed the human-caused crisis.

Linking to globalclimatestrike.net, where activists are registering events for the week of action, 350.org also shared a video Thursday and encouraged others who intend to join the climate strikes to make their own.

The Global #ClimateStrike is in just 8 DAYS. On Sept 20th, millions of people worldwide will walk out to demand an end to the age of fossil fuels. Share this video (or make your own) to let folks know you're out of office. Then RSVP here: https://t.co/MTIAHxZ9sN#StrikeWithUs pic.twitter.com/Ez54COvYdJ — 350 dot org (@350) September 12, 2019

"This September, millions of us will walk out of our workplaces and homes to join young climate strikers on the streets and demand an end to the age of fossil fuels," read the organizing website. "Our house is on fire—let's act like it. We demand climate justice for everyone."