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...