Mark Wegner says there were no warning signs of what was about to happen to the 12-year-old girl. She was stabbed 19 times in a Waukesha park last Saturday.
Wegner tells WISN-TV all three girls were friends and were "just normal students" who participated in class. The principal says the crime has been difficult for everyone at Horning Middle School to comprehend.
A criminal complaint charging the girls says they were trying to pay tribute to the fictional character Slender Man when they lured the victim into the woods and stabbed her. The complaint says the girls believed the character was real.