It is called "Bokura no Himitsu wo Kyouyuu Shiyou ka" by Shiraiki Yuki. It's a oneshot (actually, a collection of oneshots) There is also a sequel oneshot called "Bokura no Himitsu wo Kaihou shiyou ka"
I did this in first grade and have always wanted to try it again. I thought maybe I had been remembering wrong all these years. We have a project for this week. straw weaving diy (great project for kids)