You can use a medium weight double sided fusible attaching the same color to the front and back of leaf shapes and have kids sign them (the pointy oval types work really well). Then stitch down the center from one point to the other. Another nice shape for leaves on something like this are hearts. Then your leaves will have dimension and you can then appliqué a really cool trunk of the tree. Perhaps even having additional leaves on the ground of other staff members or even the kids parents that may have helped in the classroom. (My parents are both retired teachers and I've made my share of retirement quilts.....I like pigma pens but sometimes they are hard for kids to write with and I end up using a sharpie.)