H.W.
What about a temporary tattoo? Novel, it will stay on him, and it won't interfere with the teacher's rules about not bringing toys.
When I am teaching (and I have the same rule), the kids know that if they bring a toy, I keep it until they are picked up and then hand it to the parent. They may or may not have been 'in on' the toys in the backpack. And really-- I don't know about the toys unless the kids take them out. I've had a few parents not support the no-toys rule (which we have for very good reasons) and then I was clear at drop-off that they needed to take those items home. I think the tattoo wouldn't be the same sort of distraction that a toy would be.