Hribal has been charged as an adult with four counts of attempted homicide, 21 counts of aggravated assault and a weapons charge following Wednesday's stabbing spree in Murrysville, a suburb about 15 miles east of Pittsburgh.
"I think he understands what he did," attorney Patrick Thomassey said in an interview on "Good Morning America." "I don't think he at this point understands the gravity of what he did. I don't think he realizes how severely injured some of these people are."
Thomassey said he wants to have a mental health expert evaluate the 16-year-old, noting that any defense he will offer will likely be on the boy's psychological state. He said Hribal did not have a history of mental illness and was not aware that the boy had been bullied.
Authorities say Hribal ran down a hallway at Franklin Regional High School with two kitchen knives, stabbing and slashing 21 students and a security guard.
"He wasn't saying anything," said student Nate Moore, who was slashed in the face. "He didn't have any anger on his face. It was just a blank expression."
Hribal stopped only when Sam King, an assistant principal, tackled him. The stabbing rampage lasted about five minutes.
At a brief hearing Wednesday night, District Attorney John Peck said that after he was taken into custody, Hribal made comments suggesting he wanted to die.
The teen is being held without bond in the Westmoreland County juvenile detention center.
Thomassey is hoping to have the case moved to juvenile court. Under Pennsylvania law, he will have to convince a judge that Hribal can be rehabilitated in juvenile court, which would have jurisdiction over him until he's 21. If convicted as an adult, Hribal faces likely decades in prison.
ABC News and The Associated Press contributed to this report.