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 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.