It is. It's actually addictive. It gives your brain a dose of dopamine, which is basically a chemical high. So you want more, and more, and suddenly you are addicted to getting a "like" for something. And when you don't get a "like" for something, it can cause anxiety or even depression.
Are those really things we want to subject children to even more than already happens?
There are lots of concerns to have about social media in general. I don't think this specific instance is the one to be most concerned about.
I can see people fighting their kids being involved with this site, but then letting them have their own smart phone a few years later.
It's not a bad idea to question things like this, but if they are shown to be on the level I think it's ok to trust the school to some extent.