I suppose the school must have good reasons for forbidding the wearing of jewellery. So we should obey that.
Meanwhile, scapula is not jewellery, nor is it metalic. It is a piece of garment, similar to an under shirt. I’m sure if this is explained to the school, they can understand it. And if they insist that it is not permitted, then you can also sew the scapula into the other garments your child is wearing, or make a scapula big enough to be the same size of an undershirt.
As for the miraculous medal, it is also not jewellery. However, since it is usually metallic, they may not be able to make an exception. But miraculous medals do not need to be worn. You can just keep it in your pocket or even in the wallet. And it does not need to be metallic. It could be drawn onto a piece of garment indeed.
What is important is not this scapula or that medal, but the practice of the devotion. If this or that physical appearance does not meet the rules, then make it into another form where the rules don’t apply.