The would-be jihadist, aged just 15 at the time of her arrest in January 2016, had tried to contact Isis group leaders on Twitter but was arrested after her parents became suspicious that she was experimenting with chemicals in the basement of the family home.
A court in the town of Holbæk, 65 kilometres west of Copenhagen, on Tuesday found the girl, who is now 17, guilty of "attempting a terrorist act".
The unnamed girl, a Muslim convert, had also written notes about planning to carry out the attacks on both her former primary school and a Jewish school in Copenhagen.
Prior to sentencing Thursday, prosecution lawyers had advised she be jailed indefinitely, an option in Danish law if a person is found guilty of a very serious crime and judged to be a risk to society.
The girl's defence lawyer Mette Grith Stage said that her client was relieved over the six-year sentence.
“She is extremely happy, this has been a huge burden for her to have on her shoulders. Particularly the indefinite sentence recommendation,” Stage told Politiken.
Senior prosecutor Kristian Kirk said that he was surprised by the sentence.
“The High Court has clearly stated that the basic sentence for terror attempts is 12 years, and as I stated during proceedings, some reduction should be given, but to give a half-sentence compared to 12 years has certainly not been seen before,” Kirk said to Politiken.
Kirk had requested a ten-year sentence should the court not agree to meet the prosecution's request for an indefinite sentence, reports the newspaper.
The prosecution lawyer said the he was considering an appeal against the sentence.
On Wednesday, broadcaster TV2 released a remarkable recording of the girl talking to her former school teacher prior to her arrest.
In the recording, the girl says that she is planning to bomb her former school in Fårevejle during a prom party and claims that she spoke to Isis leader Abu Bakir al-Baghdadi on Twitter.
She adds in the recording that she would not carry out the attack on the school unless she was ordered to and that she cannot go to Copenhagen to carry out an attack because she would need to ask her mother for permission to take the train.
Prior to Tuesday's verdict, Kirk presented a psychological assessment of the girl that concluded that she was mentally fragile, insecure and had identity problems, reports Politiken.
The legal medical council assigned to the case recommended that she be given an indefinite sentence until she is healthy.
But this would have been an overreaction, argued Stage, who recommended a prison sentence of six to eight years.
The amateurish nature of the girl's preparations for the attack were also taken into account by the court, reports Politiken.
Nine of the 12 jury members voted to follow Stage's recommendation, two voted for indefinite sentencing and one advocated a ten-year sentence.
Stage praised the jury after sentencing Thursday.
“The result reflects the fact that the court did not feel bound by the Legal Medical Council report, and that it is neutral and independent. The Legal Medical Council recommends one thing, but we as a court must make a judicial evaluation, and we are not bound by anything other than our own convictions. That is very satisfying,” she told Politiken.
The lawyer added that she believed the sentence was also an expression of faith in the system's ability to help the girl to reform.
“The authorities have her in their hands for the next long period of time. All options to begin a process of anti-radicalisation are available, if it is concluded that this is necessary. In any case, [authorities must] bring her into a context in which she is influenced by sensible people,” Stage said.