The next thing is to get the whole class involved.  Collect tubes from kitchen rolls, toilet rolls etc.
Decide what sort of size you want the leaves to be.
Get everyone to make some thin tubes by rolling newspaper with a smear of paste (flour paste created by bringing some flour-mixed-with-water-to-a-thin-cream to boiling will do since this is likely to be a very temporary thing) and dried thoroughly.
Get everyone also to make a pile of leaves out of green paper (even green bits glued back to back out of pictures in magazines).
You then have your raw materials and can assemble them with glue, wire and by applying one or more layers of paper laminate over them for unity and strength.