Many real world distractions exist. Drugs and alcohol exist, and will be part of many of these children's lives. Just because something exists in the real world doesn't mean it belongs in schools.
Frankly, smartphones should be discussed in health class, much like drugs and alcohol, and in a similar tone.
It's not really smartphones by themselves that need to be discussed, but rather (simplified) the dopamine loop reward system.
Explain it to young kids as the smartphone giving you a 'treat' for doing nothing. Eventually you get lazy and won't do any work because you get a 'treat' from the smartphone for free whereas if you play sports or hang out with your friends you only get the 'treat' for doing something.
Then explain that very smart people have taught the smartphone how to make the 'treat' tastier and tastier until you spend most of your time chasing treats instead of doing and enjoying things.
Frankly, smartphones should be discussed in health class, much like drugs and alcohol, and in a similar tone.