Sentenced when she was 15 years old for her plot to attack schools with bombs, the IS sympathizer has stabbed an educator while in jail. Medical advisors recommended she be incarcerated for the foreseeable future.
A high court in Denmark extended to eight years the sentence for the now 17-year-old Muslim convert.
The individual, who cannot be named under Germany's media code, was arrested in her home village 65 kilometers (40 miles) west of the capital, Copenhagen, in January 2016. Her parents had alerted police to suspicious chemical experiments in the basement of their house.
Bottles of hydrogen peroxide, citric acid, acetone and a plastic tray with unknown liquid residues in a metal bowl were found by police. While not enough to build a dangerous bomb, according to investigators, the girl had also written notes about her plan to target her own school as well as a Jewish school in Copenhagen.
She also wrote about her sympathies for the "Islamic State" (IS) group and tried to contact its leaders via Twitter.
After her initial detention she brutally attacked a youth warden, stabbing him repeatedly with a piece of broken mirror.
A district court had sentenced her to six years in May, saying that the ruling took into account "the nature of the accused's serious crime" and her age.
Medical professionals told the court that the girl was so dangerous she should be jailed for the foreseeable future.
However, the court opted for a time-limited sentence, increasing the initial six-year term to eight.
jm/msh (dpa, AFP)