While most of the books in this post are recommended more for individual and group counseling, this is one I can absolutely see using in class lessons (I’m Stretched, I Can Say No, and The Whatifs as well).
Here are the messages in this story: what ifs can be positive, lots of people have self-doubt, sometimes our brains lie to us (we can have what ifs that contradict the evidence around us), sharing our worries is helpful, and you can support friends who worry. I’d say it’s a great fit for 1st-3rd.