Heavenly Creatures (1994) on Netflix

Pauline Parker and Juliet Hulme met in school during the 1950s. Instant best friends, they proceeded to spend every minute possible together, often writing about a fantasy land of their own invention. More and more estranged from their respective families, the two girls realise that they are extremely different from most other people, and agree to take any steps necessary to ensure that they are not seperated. The two families are increasingly concerned about the girls' friendship in a strictly moralistic era.