The wounded - who are all believed to be between 14 - 17 years old, have been taken to hospital after the incident today.
Four of the students have sustained 'significant' injuries and were airlifted from the scene.
The suspect, who is believed to be a male student at the school, is now in custody according to a News report from ABC.
School officials published this bulletin on Wednesday morning: 'A critical incident has occurred at the high school. All elementary schools are cancelled, the middle school and high school students are secure.'
The attack lasted for 30 minutes and is believed to have begun at the start of the school day.
A spokesman for Westmoreland County emergency management said the students were injured in 'numerous classrooms and hallways' of the high school this morning.
The US has suffered several mass attacks in recent years - with the Sandy Hook massacre in 2012 causing many to call on states to tighten their gun restrictions.