Poison for the Fairies (1986)

Storyline:

Poison for the Fairies (1986) Mexico City circa 1965: Flavia is an aristocratic little girl, who is very lonely and bored. At school she meets and befriends a strange and beautiful girl named Verónica, who dreams of becoming a witch. Their games get increasingly sadistic and macabre, and escalate to involve the bloody murder of a piano teacher and macabre mayhem.