I think you want to use unreliable or undependable rather than unstable. Example:
Your friends can be unreliable. Your parents are more dependable.
When talking about your marks at school, say grades instead of scores.
- Our parents understand us best.
- They have often been through the same experiences, so they can give us advice.
- I am afraid to tell my best friend my secrets because he might tell somebody, but I don't think my mother would do that.
- Although you might complain about your grades to your friends, they might not be very sympathetic.
- Few really care for others.
- You can count on your parents. They stand by us and give us good advice to help us solve our problems.
- My mother encouraged me to keep trying, and she told me that I was learning despite my bad grades.