Help me find a book i read a long time ago?

It was about a girl who walked to school every day and one day there was this other girl who was walking to. The other girl always looked at the sky and convinced the girl she was a witch. They became friends and the girl was going to make this spell so she gave the girl a rock with string that she could not cut. I believe It ended while the non-witch girl was not sure if the witch girl was actually a witch. Help me find this book pretty please! I really wish to read it again.