The pouches work really well.
It means the DC are responsible for their own phones - no safe with lots of phones in.
Not sure I understand your comment about the pouch having bits of random metal in them?!
The pouch becomes part of the school kit, and the DC have to have one even if they don't have a phone, so they can't say "I don't have a pouch".
If they lie about having a phone on them then they will get into trouble if caught, same as any other school rule.
It's worked well in my DC's school. You're always going to get the odd one or two who are determined to break the role, but it has changed the culture of phone use overnight.
Before the pouches were brought in, DC as young as year 7 were routinely being shown unsuitable content including hard core porn by other DC at school.
Now, that simply isn't happening.
How can an 11 year old possibly process hard core porn without it being damaging to them? The government and the schools had to do something. Leaving it up to the parents and tech companies wasn't working.