Cognitive load is a well-known and established theory. Basically, it posits humans have a limited amount of cognitive processing power available at any moment; game theorists would declare our cognitive capacity is a zero-sum quantity. What is used for one purpose is unavailable for other purposes.
In classroom, we want to limit extraneous cognitive load which is wasted on activities unrelated to the task at hand. Understand the task at hand consumes more of the cognitive load, and the rest is available to create new knowledge.
Consider for a moment cell phones in classrooms. Surely these devices will increase the extraneous cognitive load. Student attending to the screen cannot attend to their work or the learning. When they are processing the messages, they are not processing math, science, or any other content. When they are socializing about those messages, they are not interacting over the content we intend.
The obvious step to take, then is to ban cell phones in classrooms. It is reasonable to assume students’ will no long expend cognitive load attending to those devices.
Consider what is more likely to happen, however. Students will continue to use their phones, but they will expend cognitive load, and a lot of cognitive load, trying to avoid being caught. As a teacher who started far before cells phone arrived, I have seen the change in attention. I have seen students briefly distracted as they checked a message. I have seen them take a few seconds to find a new song. I have seen it all.
I have also seen the distraction of trying to avoid being caught. I have seen the one whose phone has been taken away distracting others to have them send messages on their behalf. I have seen students distracted for long minutes as they schemed how to get their phone back or just fumed over having lost it.
The conclusion I have drawn is that teaching responsible cell phone use… and allowing it when appropriate is far less distracting than the distraction of confiscated cell phones.