Just give them an 'F'
For those too stupid to realise that they are there to learn not send cat piccies to their friends.
The School District needs to make a policy.
Use a phone in a lesson and you FAIL the course. That will screw up the GPA's for those little darlings/cherubs wanting to go to Harvard Law and spend the rest of their lives screwing the rest of us (or get a 1st Draft pick for any of the sillier US Sports)