Middle grade standalone chapter book that I read at some point in the 90’s, in my teacher’s classroom library. This was a parochial Catholic school  and I was surprised and delighted to find something so occult. The book was blue with something white on the cover and it was a shorter title. In the book, a teenaged girl in a large isolated house in the country is contacted (via ESP? Dreams?) by a supernatural entity that is slowly revealed to be evil. It might have some connection with ice, and is male. There is some specific danger to the protagonist’s baby sister. At first the protagonist’s parents and the nanny do not believe her but at some point the nanny seems to come around. The once scene I remember is the nanny standing in a full bathtub while the tap overflows into the hallway, while holding the baby, because the supernatural entity was physically manifesting inside the house and the nanny thought the water would protect them. Its possible the evil entity lives at the bottom of the garden? I think the author was female? 
Finally, my school had a lot of books published by Scholastic.
I’ve been trying to remember this book for 30 years, I hope you know it!