The school shows that you have recently felt that you are lacking in some aspects and are learning to improve.
A tiger may symbolize someone in your life. He is as fierce and dangerous as a tiger in your heart. He tried to control you. Tigers may be the difficulties or pressures you encounter, which make you feel threatened and unable to resist.
Sister may be the simple, naive, kind and willing one in your heart. As I don't know your sister very well, I can only speculate. ...
The tiger eats her, which means that you have lost the same quality as your sister and become more suitable for your life.
This kind of growth is very painful for you.
But this dream is not bad.