Alex Hribal, a Franklin High School sophomore, faces new charges after the discovery of the note, which read "can't wait to see the priceless and helpless looks on the faces" of his fellow students.
Police also said the boy said he had "more people to kill" when a school official attacked him after the stabbings, which left one adult and 21 students injured April 9.
Hribal initially faced charges of four counts of attempted homicide, 21 counts of aggravated assault and having a weapon on school property. He was arraigned Friday on 21 counts of attempted homicide.
Hribal's lawyer, Patrick Thomassey, said he didn't understand why the note in the locker would have supported the additional charges of attempted homicide.
"We had four counts of attempted criminal homicide and  counts of aggravated assault. I don't know how much it changes, really," he said. "I don't see the point of it, quite frankly, but it is what it is."
In the criminal complaint filed Friday, police said allegedly refused to drop his knives after he had been tackled to the ground.
"The accused refused to drop the knives and was saying: 'No, I am not dropping them, my work is not done, I have more people to kill,' " Murrysville police Sgt. Charles Tappe and Westmoreland County Detective Robert W. Weaver wrote in the complaint.