Witnesses described chaos and panic as Auvinen shot dead his headmistress, five boys, two girls, and wounded a dozen others as they tried to flee the carnage.
Tuusula is a town of 35,000 around 60km from the Finnish capital Helsinki.
Miro Lukinmaa, a student, told the Iltalehti newspaper: "Suddenly people began running and shots were heard and began raining down.
"I saw injured people lying in the corridor. We started to run and followed [the crowd] in panic. Everyone was trying to squeeze through a narrow door."
The shooting came after a video entitled "Jokela High School Massacre - 11/7/2007" was posted on the video-sharing website You Tube.
The video shows a still photo of a low building that appears to be the Jokela High School.
The photo then breaks apart to reveal a red-tinted picture of a man pointing a handgun at the camera.
"He [the gunman] was moving systematically through the school hallways, knocking on the doors and shooting through the doors," said Kim Kiuru, who was teaching a grade 8 class when the shooting began.
"It felt unreal, a pupil I have taught myself was running towards me, screaming, a pistol in his hand."
Olli Laine, a police officer, earlier said that police had evacuated students from the building.
"This is a peaceful place, nothing like this has happened and nothing like this is to be expected either," Hannu Joensivusaid, the mayor of Tuusula, said.
Despite having the world's third-largest per capita handgun ownership, violent incidents are rare at Finnish schools.