It depends entirely on the 15 year old, but as a parent, if my child is not at his or her academic grade level, I'd want to know why. If an eleven year old isn't reading at the fifth grade level, and the parent doesn't care, that's a red flag to me. The truth is, there are eleven year olds who are slacking if they're not doing work well beyond their grade level, and eleven year olds that nobody is going to be able to have at grade level within a year. It depends on the child.
I know a young man, though, who struggled through reading all the way through grade school, in spite of his obvious intelligence, only to find at the age of 14 that he had a problem with the muscles of his eyes! All those years of academic struggle, because his problem hadn't been correctly diagnosed.
It is necessary, too, to take the parent's own educational attainment into account. Have the parents been well able to provide for their family with less than a high school educations? I don't think that is a reason to believe their children will be so fortunate, but surely it is a reason for understanding why they could care very much about their children and yet not be at all alarmed about their lack of academic accomplishment. Still, I find it hard to believe there is a parent out there who does not know that an adult needs to be able to read and do math like one, in order to read contracts and manage personal finances and the like, if nothing else.
Likewise, there are parents who are so trusting of educational "authority" that one opinion that puts a low expectation on their child's chances of academic success is all they need to give up on the child. As much as I would argue they not give in so easily, I really don't blame them for that.
These are all reasons to have a conversation with the parent, though. The leap to concluding that there is neglect is something else again....and, actually depends on the laws of the state. If the citizens of the state have defined no legal responsibility on the parent beyond what the parent is doing, "reporting" the parents is only going to cause strife and anguish. As much as it might pain a person to come to that conclusion, there is no point to it.