no measurable impact on grades in the US from locking phones in pouches, with measurable decrease in cell activity: https://www.nber.org/system/files/working_papers/w35132/w351...

school phone bans in brazil appear to increase test scores by 0.06 SD (although the identification strategy is a bit sketchy): https://www.nber.org/system/files/working_papers/w35233/w352...

I was thinking the same thing, the outcome they're measuring is unrelated to the reason for the instituted change.

Study: Schoolwide phone bans show no significant impact on road death toll.

> I wouldn't have expected any impact on mental health resulting

AFAIK declining mental health was the main driver of the bans - put forward by its proponents like Jonathan Haidt, author of "The Anxious Generation."