"A critical incident has occurred at the high school. All elementary schools are cancelled, the middle school and high school students are secure," said the Westmoreland County school district in a statement posted on its web site.
A 16-year-old boy - identified by authorities as Alex Hribal - was arrested after the incident at Franklin Regional High School in Murrysville, 40 km from Pittsburgh. Police said he is the attacker and a student at the school, the spokesperson for the county's Emergency Department, Dan Stevens, told reporters.
The teenager, Alex Hribal,was arrested after the incident
He added that the attack began shortly before the first class of the day, lasted only minutes and the attacker wielded two knives 20 and 25 centimeters (8 and 10 inches) long.
The suspect was later questioned by police and taken to a hospital for minor head injuries.
Initial reports broadcast by local television say that the attacker went through the high school's halls acting "crazy" and stabbing as many students as he could before he was overpowered.
Several of the injured were taken to medical centers by helicopter with eight of them being transported to the Forbes Regional Hospital in Monroeville with stab wounds, Pittsburg's Channel 4 TV reported.
Forbes Hospital physician Mark Rubino said that three of the students required surgery and two are in critical condition. Other medical authorities said that at least seven people were seriously injured with cuts and penetrating wounds in the chest and abdomen.
US President Barack Obama sent a message of solidarity to the relatives and victims of the attack from Texas, where he is attending the funeral services for the victims of the shooting attack last week that left four people dead and 16 wounded on the Fort Hood military base.