My DS has lost so many things, mostly jackets, sweaters, lunchbox, one croc (!), socks, you name it! We have mostly found everything again. Just today he left his book in aftercare he says. It will likely be found tomorrow. I'm not sweating it too much anymore. I would also make sure not to let him take things to school that are too upsetting if they are lost; that's what I do with DS. It's kind of sad, because DD never looses anything and is allowed to bring to school pretty much whatever she wants.