Pearl in the mist - V. C. Andrews

Fate has whisked Ruby Landry from a simple life in the Louisiana bayou. But her new riches bring more treachery than happiness.
After a year spent living in his splendid New Orleans mansion, Ruby is still rejoicing in the love of the father she had never known. But she has some venomous enemies in the household: her stepmother Daphne, who sneers at her backwater upbringing, and her twin sister Gisselle, who is full of bitterness and backstabbing.
Ruby hopes she will find some peace at the exclusive girls' boarding school her father has chosen for her. But the legendarily strict headmistress, Mrs Ironwood, plots with her stepmother to make Ruby's life a misery. Still Ruby doesn't lose hope - until a terrible tragedy leaves her all alone, forcing her to summon every last ounce of strengh to reclaim her home, her future and the happiness she once knew.