Snow, The (Point Horror) by Caroline B. Cooney - Vintage Paperback

Outside, she'll freeze...
Inside, she'll die... 

Christina has survived her first semester away from home, overcoming more than just homesickness. She has survived the Evil. Until now.

Now winter has arrived, with screaming winds and blinding snow. The cruel Shevvingtons, with whom Christina and the other children board, have already destroyed beautiful Anya, reducing her to a helpless creature, an empty shell. They have selected their next victim. Christina's dearest friend. Sweet, trusting Dolly. Working with the Shevvingtons is someone sinister who lurks in the basement, laughing insanely.
Is there any chance that Christina can revive the old Anya and also save Dolly?

But first, Christina must save herself. . . .