I've seen this before. To quote a barrister friend of mine, regardless of the law, it mostly depends on how big a stick the person having their device confiscated wields. The stick in this case is connections, parents etc. They rely purely on submission to enforce this policy. A simple "no" removes consent and their ability to do anything about it. If after that it is forced upon you, then things get "interesting" to quote him because it depends how they try and enforce it.

For example the barrister friend in question's daughter had her phone confiscated and it went missing in custody of the staff. The school disclaimed all responsibility but paid the moneyclaim out for a new phone quietly when the relevant stick was wielded and removed their policy of confiscation immediately as they worked out it was a liability. Turns out that while the law says they can do this, it doesn't mention anything about having no duty of care of other people's property at the same time...