A second teenage boy has been charged after a school in regional New South Wales was set on fire.

Police initially charged a 17-year-old boy on Monday night who is alleged to have entered the grounds of Cowra High School and set fire to the administration block and the library, causing extensive damage to both.

WATCH THE VIDEO ABOVE: Teen arrested after allegedly torching Cowra High School

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“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.”

Detectives will allege he livestreamed the act on social platform Snapchat, with a running commentary on the fire.

He allegedly could be heard saying: “Look at that s**t burn, boys.”

The 17-year-old boy allegedly livestreamed the blaze on social media. Credit: 7NEWS

Damage to the school is extensive. Credit: 7NEWS

7NEWS has been told the father of the accused firebug called police after seeing the video and recognising his son.  

Police have charged the 17-year-old boy with arson, breaking and entering, as well as disseminating footage of the alleged offence, known as “post and boast”.

Police have since arrested a second boy, aged 14, outside Cowra High School on Tuesday afternoon.

He was taken to Cowra Police Station and charged with damage property by fire/explosive more than $15000 and aggravated break and enter serious indictable offence in company – not steal etc.

The 17-year-old boy was refused bail by the Children’s Court and remains behind bars.

The 14-year-old boy was also refused bail to face Children’s Court on Wednesday.

Students, who were due to return on Tuesday for the start of term three, were told to stay away and diverted to home learning.