Gwent Police say the 15-year-old girl from Newbridge was arrested on suspicion of threats to kill, possession of a bladed article on a school premises and conspiracy to commit murder.
The 14-year-old girl from Risca was arrested on suspicion of conspiracy to commit murder.
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Rain clipping the far north through the evening but elsewhere staying dry with some sunny spells.
Public Health Wales figures show 25 more cases of the virus have also been confirmed.
Photos taken on Saturday morning show Roald Dahl Plass strewn with empty beer bottles and discarded canisters of laughing gas.