Good question! There is nothing legal about running away at this age. Most often a 15 year old that runs away will be brought back home if they are found by police. They may interview your parents or sometimes social services are called and they may interview the family to make sure that there is no sign of child abuse.
As for consequences, it is up to the officer. They may warn you that this can be put on your police record. They may make suggestions to you and your family about other consequences and possible suggestions to prevent this from happening again.
Before you consider running away, reach out and talk with someone about what is going on. Running away can be scary and put a person in more danger and unhealthy conditions.