The "random spot checks with handheld scanners" seems a bit excessive to me. They're phones, not weapons. Airport style security measures against them is hardly necessary.
I would think it's more reasonable to just say that phones should be kept switched off and out of sight at all times during the school day. If any member of staff sees them being used, or if they're heard ringing during lessons etc. they can be confiscated and handed back at the end of the day.