Why don't I just refuse to go to an English competition in order to stay with my grandma?
Oh, this is not really true is it? You would do that? Why would you rather stay with your grandma?
My parents are gonna force me to go to the competition and I don't want to go.
If you are suppose to go to this for school you need to be there. Its part of learning and getting responsibilities. You should do what you are suppose to do.
It's my life and I can do what I want. I already made my choice.
Your parents just want the best for you. Making an excuse won't help you out in life. Sometimes facing what we need to is good in the long run for us.