Police charged a 17-year-old male with arson, breaking and entering, as well as disseminating footage of the alleged offence, known as "post and boast".
Emergency services were called to Cowra High School, in the NSW central west, about 7pm on Monday after reports of a fire.
Firefighters extinguished the blaze, which was confined to the school's administration block, although the building was badly damaged.
"It's a wholly dangerous act," Acting Inspector Adam Tonkin told reporters on Tuesday.
"Doing anything with fire and especially posting it, we consider that a fairly serious, flagrant disregard of the law."
The alleged video circulated on social media by the accused offender helped form part of the investigation, Insp Tonkin said.
Deputy Premier Prue Car, who holds the education portfolio, described the arson as a "really disgusting act".
"I'm really sad for Cowra and we will do everything possible to get their education back up and running," she said.
Other buildings at the school sustained smoke damage but no one was injured, police said.
Officer arrested the teenager at a home in Cowra about 10pm the same evening after he allegedly filmed himself starting the fire alight and uploaded the footage to social media.
The teen was believed to be an ex-student at the school, police said.
He was refused bail to appear before a children's court.
Cowra High School would not be open to students on what should have been the first day of term three, the school said in a post to parents on its Facebook page on Monday night.
"The safety of our school community is our number one priority and for this reason our school will revert to learning from home tomorrow for all students," the post said.
Monday's fire is the second at a major regional high school in NSW in the past year after Deniliquin High School in the Riverina was targeted in November.
Two boys were charged after 30 classrooms were destroyed in the Deniliquin blaze, forcing a major rebuild and disrupted learning for about 500 students.
''Post and boast'' laws were introduced in 2024 to target performance crime by imposing a maximum two-year jail term but critics say there is no evidence harsher penalties stop the conduct.
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