The two adults and a child all suffered knife wounds in the incident at Ysgol Dyffryn Aman in Ammanford, Carmarthenshire. Dyfed-Powys police said they were also investigating alleged threats by a 15-year-old boy that alluded to the incident.
Police went to an address about five miles from the school and arrested a boy. The force said its detectives were working to establish if there was a link between the two incidents.
All three people who were injured have been discharged from hospital and are recovering at home.
Speaking outside the locked school gates last night, Supt Ross Evans said: "A 13-year-old girl was arrested at the scene. She has now been charged with three counts of attempted murder.
"[On Wednesday] evening, our control room received a number of calls reporting concerns over messages being shared on social media, which had references to the incident at Ysgol Dyffryn Aman.
"Officers swiftly carried out a warrant at the home of the person believed to be responsible for this content, and a 15-year-old male was arrested. He also remains in police custody while we investigate these allegations."
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