There is a large and growing body of evidence indicating that pervasive phone connectivity has led to large increases in psychiatric issues among all demographics, and particularly among younger people who have now grown up immersed in a phone culture.
More informally, smartphone usage by children promotes a short attention span, a lack of any sense of presence where actually situated in the physical world, as well as less and lower quality interaction with others, leading to poor social skills, anxiety, social isolation, and a focus on superficial social signaling over meaningful human interactions, ultimately producing the mental illnesses referenced above.
I appreciate that there is a body of research discussing possible implications of large-scale phone connectivity, but this does not meet the bar of "objectively harmful externalities".
I'm not even talking about methodologies or replication or evidence or p-hacking (all of which are huge challenges to this sort of research).
On a much more fundamental level, the statement "unauthorized smartphone use in a classroom setting is objectively harmful" is a defensible statement. I don't need a study to tell me that, nor should anyone. The extraordinary claim in this case would be the opposite, for which I would have to see tremendous evidence, and which even then I would likely not believe.
More informally, smartphone usage by children promotes a short attention span, a lack of any sense of presence where actually situated in the physical world, as well as less and lower quality interaction with others, leading to poor social skills, anxiety, social isolation, and a focus on superficial social signaling over meaningful human interactions, ultimately producing the mental illnesses referenced above.
See e.g. https://kagi.com/search?q=summary+of+mental+health+outcomes+... -- there are far too many sources to even list here.