What do you mean? Here's one published last year:
> This study does not provide evidence to support the use of school policies that prohibit phone use during the school day in their current form, given that no differences were observed in mental, physical, and academic outcomes for adolescents attending schools that permit, versus restrict phone use. However, the negative associations found between increasing time spent on phones/social media and worsened mental health and wellbeing do provide evidence on the need to address phone and social media use in adolescents, and school policies should be developed as part a more holistic approach. [...]
https://www.thelancet.com/journals/lanepe/article/PIIS2666-7...
However this study in particular is pretty bad, see the comment elsewhere about the screentime not spent not going down (possibly cause of extant classroom bans)
In this case the prevailing theory is that the phones were already banned in classes, so the schoolwide ban didn't do anything to screentime. Which, duh.
I’m a strong supporter of following the science. But this is a hard thing to measure. And the effects may be fairly diffuse or accumulate over time.
In order to quantify whether something is too much, you need to quantify what an optimal or better baseline would be.