I promise this old man diatribe has a point...
There were no smart phones when I was a kid. They were about a decade away from being mainstream consumer devices when I graduated high school. If you wanted to know something, you went to the library and looked it up (which almost nobody did!). If it wasn't taught in school, shown on TV, or told to you by your parents, it didn't exist.
We still managed to be distracted and bored in class, we just didn't have as many options for dealing with it and ended up paying marginally more attention as a result.
A connected smartphone is an awesome brain augmentation device if used correctly - to look up things in the moment you want to know them. Kids will be kids, though, and they use them for games and social media to alleviate boredom and to prey on each other like the socially inept little bastards they can be.
Don't block mobile phones. Block mobile data and provide school WiFi that only permits approved websites.