The small high school, located in Marinette - just an hour north of Green Bay - was closed Tuesday, one day after the15-year-old student held 23 students and a teacher hostage for several hours.
The ordeal ended with the hostages being released unharmed and Hengel shooting himself.
The hostages said they tried desperately to keep the gunman calm by chatting and laughing with him.
"I didn't even think it was actually happening until I actually had figured out some of my friends were missing," said a student.
The standoff began during the school day's final period.
Hengel apparently left the classroom and returned moments later with a backpack which held at least two handguns. He pulled out one of the guns, and students say he shot the movie projector and ordered his classmates to hand over their cellular phones.
Terrified students kept Hengel calm by talking about hunting and fishing. It all ended well after dark when police busted down the classroom door.
"As the officers approached him, the suspect then fired one shot and injured himself in a self-inflicted gunshot wound," described Marinette Police Chief Jeff Skorik.
The hostages were immediately evacuated from the room.
The boy's motive remains a question.
School officials are now dealing with the emotional scars. School officials plan to offer counseling for students.
The Associated Press contributed to this report.