This study was done on college students, not elementary and HS students, college students at Cal. There are a lot of caveats here and trying to apply them to elementary education is good, but outside the scope.
Aside: If you really want test scores to improve, give the kids food. My SO is an educator and she's had kids come in that only get food at school. All weekend they may only eat a bag of Cheetos, sometimes up to 2/3 kids on Monday mronings. School breakfast and lunch are sometime the only food they get.
https://www.brookings.edu/blog/brown-center-chalkboard/2017/...