She also had an appointment with a psychologist for violent threats made last week, according to daily Express.
Sixteen-year-old Tanja O. is still missing after another 17-year-old student was able to stop the arson attack when she happened upon her in the girls' bathroom at Albert Einstein high school in Sankt Augustin, some 10 kilometres from Bonn, around 9 am.
“She had a tear gas pistol, 10 Molotov cocktails, a flame thrower and five kilogrammes of gunpowder,” the paper reported, adding that the girl's rucksack also contained a suicide letter to police.
But Tanja O., who was also reportedly wearing a mask, may have been startled in the bathroom by the other student. She then slashed the other girl with a knife and nearly severed her thumb.
Screams alerted teachers to the incident and they called the police to tell them the school was under attack. The injured girl was transported to a Bonn hospital for emergency surgery.
Authorities evacuated some 800 students from the school to a neighbouring athletics building. According to local daily paper Kölner Stadt-Anzeiger, the attack sent students fleeing from the school in panic as worried parents rushed to the scene.
Police are now searching for Tanja O., who they believe escaped on foot. Authorities have already searched her home, confiscating her laptop.
Just last week the girl threatened to attack the school, for which she was scheduled to meet with a school psychologist on Monday.
Germany is particularly sensitive to school violence after 17-year-old Tim Kretschmer killed 15 people at his former school with his father's gun in the southwestern German town of Winnenden in March. He later killed himself during a shootout with police.
The massacre stirred up debate about stronger whether the country needs stronger gun laws or a ban on violent video games.