Sentence:  The sight of their own blood in the white snow may even have been what provided the jolt to transform them from playful children recklessly delighting in the surprise of an unseasonable snowfall into a whooping army of mutineers urged on by a tiny cadre of seditious underclassmen to turn their rambunctious <WSD>frivolity </WSD> into stunning mischief and, with an outburst of everything untamed in them (despite regular attendance at chapel), to tumble and roll and skid down the Hill through the deep snow and commence a stupendous night out. 
Meaning of the word marked with <WSD> tags:  state of being frivolous