The would-be jihadist, aged just 15 at the time, had tried to contact IS 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.
Investigators found bottles of hydrogen peroxide, citric acid and acetone and a plastic tray with unknown liquid residues in a metal bowl.
A court in the town of Holbaek, 65 kilometres (40 miles) west of Copenhagen on Tuesday found the girl, who is now 17, guilty of "attempting a terrorist act".
Experts said the chemicals she had gathered were insufficient to build a powerful bomb but the court underlined her criminal intent and motivation in a ruling.
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.
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