Is this a serious problem?

My daughter is two grades behind in school. At a young age she was exposed to a lot of things growing up and i was in and out of the hospital going through post- pardon depression when i had her little sister. That affected her school life and her attitude towards school. Her teachers held her back because of her attitude and she just couldn't focus in school.I was wondering is it normal for a 15yr old to be behind or is this a serious problem?