MURRYSVILLE, Pa. -- Teachers returned to the Pittsburgh area high school where 21 students and a security guard were stabbed.

The teachers attended a special counseling session this morning at a church before they went to Franklin Regional High School.

Students and their parents will be able to visit the school Tuesday.

Classes begin again on Wednesday, a week after the attack.

Four students are still listed in critical condition. 

A 16-year-old allegedly attacked his fellow students with a pair of large kitchen knives. He is facing four counts of attempted homicide and 21 counts of aggravated assault.

Investigators are still working to figure out why he did it.