The girl, 14, faces the charges after a stabbing attack on April 24 sent the Amman Valley School in the small south-west Wales town of Ammanford into lockdown.
Teachers Fiona Elias and Liz Hopkin, as well as a pupil, were hurt in the incident. They have since been released from hospital.
The defendant, who for legal reasons cannot be named, faces seven charges in total: three counts of attempted murder; three of wounding with intent; and one of possession of a bladed article.
She appeared at Swansea Crown Court on Monday to give her name and enter her pleas.
Read more: Two teachers injured in Ammanford school stabbing break their silence as they thank public for kind messages
She denied the attempted murder charges but pleaded guilty to the lesser offence of wounding with intent.
The judge said she would face trial for the attempted murder charge in September. The court heard the attack was captured on CCTV.
The teenage defendant, wearing a white shirt, black tie and waistcoat, stared at the ground as the judge spoke.
The trial was scheduled for September 30, with a pre-trial review on September 17.
The defendant will be remanded in youth detention until then.
Helen Randall appeared on behalf of the prosecution and Caroline Rees KC appeared on behalf of the defence.
