One of the concessions of all education is that nobody is ever fully-engaged. If you are teaching more than 1 person something, then you are dividing your attention between multiple students and hoping they understand well enough to continue. Busywork is even worse; you're just handing kids a set of guidelines and hoping they can intuit the right answer when you grade it.

It feels obvious why phones dominate this environment. If you had a 1:1 teacher/student ratio, nobody would be compelled to distract themselves because the learning is paced for them, not the average of the whole. There's time to catch up on Instagram because roll-call only requires you to answer once. There's time to check Twitter since you finished your test early. The number of places where kids are expected to simply do nothing is absurd - of course they're distracting themselves. I read Treasure Island during my mandatory poetry class, is that a reason to confiscate fiction novels from school premises for their distracting and addicting nature too? The problem is with how we teach.