Police were on hand as students arrived at the school, which is about 30 miles east of Cleveland. Some students and parents stood outside applauding as students walked inside.
T.J. Lane, 17, is facing three counts of aggravated murder, two counts of attempted aggravated murder and one count of felonious assault.
The motive remains unclear, but the prosecutor appeared to rule out theories involving bullying or drug-dealing.
Lane admitted to firing 10 shots at a group of students sitting at a cafeteria table, officials said. In addition to the three victims who were killed, two others were injured.
