It's not about looking after the property, is it? It's removing the distraction and behavioural issue during the school day and reducing how much kids come to rely on the hit they get from their mobile phones.
And tbh I think your risk analysis is way off. What's an easier target for theft by a teenager - the pocket of a blazer or a school bag left lying somewhere while its owner is having a kick around at break, or a huge clunky and very obvious box that's kept somewhere a bit more secure?
I've just dug out the letter the school sent when they confirmed the trial was being made permanent - some excerpts below.
Since the launch of the digital detox we have seen a dramatic decrease in the number of behaviour incidents relating to and/or heightened as a result of phone usage during school time. For example, fewer/no cases of students filming or messaging each other during lesson time or incidents of students being unkind via social media or distracting each other when in lessons.
^Whilst we know, it was different for us when we were growing up, young people are having to navigate through an unfamiliar, unpredictable social world, which is constantly changing. Mobile phone usage is a national concern. Our students are developing habits that allow them to understand ‘It’s going to be okay, if you do not check your phone’.
Notably, teachers have reported that they have seen a vast improvement in students' focus and attention within the classroom and this has resulted in there being a positive learning environment in the classroom.
I'm sure the school would rather just ban phones altogether to make it simpler for themselves. But they recognise that we live in a world where there are many reasons parents might want kids to have phones on the way to and from school, so this is their compromise. You can bring a phone to school, but you can't have it on you during the school day (apart from exceptions of true individual need.) If you bring a phone, this is what happens to it during the school day. Don't like it? Don't send your kid with a phone.
I know quite a few schools that are taking an similar approach now. Interestingly even the older kids I've spoken to seem to think it's a good idea.