I think the problem is more about social media than smartphones.
A recent study did a survey[1] of parents who had allowed their teenage kids to use social media. The results were pretty significant. Every parent in the survey said it was a big mistake in retrospect. Lots of problems, from addiction to depression.
That's a good distinction. I think however, in the case of phone use among schoolchildren, "smartphone" and "social media" are nearly synonymous. You just can't get a lot of the negative aspects of phone usage without tying it down to social media. (I'm thinking of your Likes, public shaming, low self esteem, distraction by notifications, photo filters, narcissism, addiction, short attention spans, difficulty concentrating, etc. It's hard(er?) to pull that off without the deep pockets of social media giants.)
A recent study did a survey[1] of parents who had allowed their teenage kids to use social media. The results were pretty significant. Every parent in the survey said it was a big mistake in retrospect. Lots of problems, from addiction to depression.
[1] I don't recall what the N was.