Bear in mind that those conclusions are for a different student population -- country of course, but also age range:

Czech schools: "The new measure, which has been at the center of heated debates in recent months, will apply in kindergartens and elementary schools, most likely from September 2027. ... It doesn't apply, however, to secondary schools, universities or vocational schools."

The Stanford study was based on "more than 43,000 middle and high schools nationwide".

Also, while half of the authors are Stanford research associates, the other half are at other institutions. The publication is a working paper from the National Bureau of Economic Research.

And from the preregistration, "Our measure of treatment (i.e., whether a given school has a true phone ban at a particular point in time) is based on organizing sales and shipping data from Yondr (i.e., a company that sells lockable phone pouches)", a company they partnered with for the study.

How .. odd. Is sales data for a single company really the right way to tell if a school has a true phone ban?