Concerning the hypothetical you've pulled out of your ass, the intelligent parent would simply call the school's office and have them notify the student. You know, the way it used to work for the 50 years or so before cell phones were the norm.
Seriously, folks, why make this such an intractable problem?
Many a person's grandfather took a rifle or shotgun to high school, either for ROTC practice or fragging some dinner on the walk home. I sharpened pencils in the hallways of my high school with the Swiss army knife I've been carrying daily for 25 years now. Somewhere along the way, we decided those utilitarian devices were more of a problem than a benefit in schools, and even suspicion of having one will get you patted down by a cop today. The cell phone has apparently become a disruptive device in the school environment, so let's accept it and move on with educating the kids.
Did parents cry foul when told their kids couldn't take boom boxes to school? ZOMG! Little Johnny won't be able to tune into the emergency broadcast system if there's a Russian attack! Of course not, that's a silly bullshit excuse, just like all the ones we're hearing now. If it wasn't a necessity before it became ubiquitous, then it's not a *need* now.
Don't like it? Pull your kids out of school and teach them yourselves. That's what I did.