Found: Book Help - Exchange Student Imposter - Thriller

I read it in Middle School in 1994.
I remember a female foreign exchange student living with an American Family.
Turns out she's an imposter. She killed her previous family I believe.
I vaguely remember Black Magic being involved. I think she needed to kill the new family by walking around them 3 times in a circle.
I think the way they found out was their other daughter traded places and reported back.
Any help would be great.
It's a teenage drama/thriller from the 80s or 90s that we read for school.

Some of these elements are in Summer of Fear by Lois Duncan, but others don't quite fit.

>2 jollyavis: That's it! I must be combining 2 stories together. This is definitely the one I was looking for. Thank you!