Yes, even in adults that particular idea is ridiculous. It is literally the old classic "If you have nothing to hide what's your problem?"
Except it's not, because this information is as easily available as if you walked next to a person yelling at the top of their lungs down a street.
The point is, like the policy or not, there is absolutely nothing that is stopping anyone, even a school board, from monitoring these people's social media posts. And there can never be a law in a democratic society making that illegal, either.
So, much as I wouldn't be terribly comfortable with a school board monitoring my kids' posts, I'm much more worried about my kid posting stuff he shouldn't be. So responsibility for NOT being a victim here lies with the person doing the posting. Unlike crimes like sexual assault, where victims have no fault role in the crime, any abuse of personal information in this case is preventable entirely by the person
not posting it for all the world to see. Sure, criminal charges exist for people who commit offenses using other people's information, public or not, and the victim in that case is not responsible for the criminal actions of another. But in general - if you don't want to be a victim of abuse, don't post information that can be used to abuse you in public. Yeah, these kids are young, but this is a lesson that parents should be imparting early (and recall that the TOU for these sites set minimum age at 13 with parental monitoring). If anything, a school board is one of the most benign parties that can be collecting this information.
Does the school board policy suck? Sure. Should it be opposed by the parents with kids in that district? Yup. However, if the school board refuses to retract the policy, there is an eminently simple solution:
Teach your kids that they should never post anything in the public areas of the Internet that can be used to harm or compromise them. If anything, the argument that "parents don't properly monitor the information their kids put online and it can therefore be abused" is a pretty reasonable justification for a policy like this simply because parents clearly aren't monitoring their kids at all in that case.
I hate the argument that "people no longer feel any sense of personal responsibility," but there are a small few areas where that really has become the reality, and the **** people post on the Internet is at the top of the list.