The girl was airlifted to Parkland Memorial Hospital in Dallas following the shooting in the town of Italy, some 70 kilometres south of Dallas, said Sgt Joe Fitzgerald of the Ellis County Sheriff's Office.
The sheriff's office reported an active shooter at Italy High School at about 7:50 AM The shooter was found and taken into custody without incident.
Fitzgerald did not have any information on the injured student's condition. Parkland spokeswoman April Foran said she could give no information.
The sheriff's office said students have been removed from the school and that an investigation is ongoing.
A message left with the Italy Independent School District was not immediately returned.
Italy promotes itself as "The Biggest Little Town In Texas" and is located just off Interstate 35 between Dallas and Waco.