I suggest you print with tempera paint or acrylic. Leaves can be waxy and
might repel the watercolor.
Use fresh, green leaves and paint the underside where the veins are to get
the full shape and texture.
One good book you might use is :HAVE YOU SEEN TREES? by Joanne Oppenheim
You could incorporate O'Keeffe THE LAWRENCE TREE painting into the lesson.
Another way to print is to place the leaf (a fresh green one) on a board,
place a piece of white muslim material over it and hit it with a mallet. The
chlorophyl bleaches the material and makes a great print. Then you have
science and art in one lesson!