You can see here that many of his sculptures end in a pair of shapes. Each builds up like a stairway with a few deviations you'd expect from a master. The basics however are not too hard to decipher. When creating these, one must always start at the bottom-most parts.
Though Calder didn't use string to make his sculptures balance, It is VERY tricky to bend the wire and make everything balance as he does. I have done it myself successfully but it took many tries. Your more experienced high school students might be able to manage it, but getting a balance point ON the loop to the next level is very precise work with no room for error. Strings however are super easy to manage. As long as they are kept short, tangles will be kept to a minimum.
As I teach the method, I have those who succeed help and teach their peers. The balance portion stumps a few but with many helpers it not a problem.
As for the shapes/forms on the ends of sticks, ANY sculptural kind of item can be used, though obviously light items are easier to manage like cardboard, foamcore, paper mache, plastics, floral foam, foamy clay, etc. The items have to be put at the end of a stick by piercing them tying them, gluing or taping. Make sure these items cannot fall off.
As for the "sticks," I have used dowels, straightened coat-hanger wire, acrylic straws from Nasco, even brazing rods if you have a metals department. Try making one yourself, and you'll see it's not as hard as you may have imagined.
THIS is the Youtube link to my video tutorial, and here is the video below.