There are no reputable studies showing both correlation and causation there.
Learning to read or to do mathematics is like anything else, it takes practice. We know, intuitively from our own lives and observes virtually all humans, that humans perform better with less distractions. It would be hard to learn how to play the Tuba with me screaming behind you. It would be hard to learn chess with a movie playing in front of you.
Phones are distractions. Less phones = less distractions = better performance, smarter kids, more likelihood to graduate, higher average income.
Numerous peer reviewed articles published in reputable journals have reported that there are advantages to banning cell phones in the classroom, yet you as the sole arbiter of "actual science" declare otherwise.
The point of science is to provide better information for us to make decisions with, not to prevent us from making decisions at all until it's done
If we waited for science to settle and provide the best information possible, we would never get anything done