The attack occurred at the Amman Valley School in Ammanford, north of Swansea, just after 11.00 am (1000 GMT) on Wednesday.
Following the incident, the school went into lockdown for four hours after a “code red” alarm sounded through the public address system.
Two teachers and a teenage pupil were taken to hospital with stab wounds.
The girl, whose name cannot be published because she is under 18, confirmed her name and address but showed no emotion when she appeared in the dock at the Llanelli Magistrates’ Court.
Senior prosecutor Michael Cray said the girl had been charged with three counts of attempted murder and possession of a bladed article on a school premises.
“Criminal proceedings are active and the defendant has the right to a fair trial,” he said.
Amman Valley School has 1,450 students aged between 11 and 18.
The girl will next appear at Swansea Crown Court on May 27.
By Agencies.
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