The girl was arrested at her home in January last year, when she was aged 15, and charged with planning the attacks after acquiring chemicals for making bombs, police said.
Police who searched the residence found a handwritten note with the words "Allahu Akbar!" (God is greatest), a date, and the address of the Jewish school in Copenhagen
A second note was marked "bomb attack on the infidels" and gave the address of her own school near Holbaek, a town west of the capital.
Police said they also found chemicals, a list of ingredients for making a bomb and a container marked "Tatp" - a common abbreviation for the explosive triacetone triperoxide.
A jury in the Holbaek district court found the girl - who is white European and was not named - guilty of attempted terrorism, the court said in a statement. Sentencing was deferred to Thursday.
The court said the girl had written two Twitter profiles in which she offered to fight for Jihadist group ISIS.
Prosecutors last month presented the court records from her preliminary questioning, stating she initially confessed to planning the attacks. The girl later changed her plea to not guilty.
Defence attorney Mette Grith Stage told Reuters her client would await sentencing before deciding whether to appeal.
In February 2015, a gunman killed two people in shooting attacks at a debating event and a Copenhagen synagogue before being shot dead by police.
(Reporting by Julie Astrid Thomsen; Editing by Jacob Gronholt-Pedersen and John Stonestreet)