What do you do when you find a in the backyard to sketch, but it flits here and there and then finally away before your students can sketch them? I have my students attend to the the butterfly to notice as many details as they can so that we can accurately identify it. We also try to notice where it is landing. Sometimes the plant it lands on can be a clue to its identity. When the butterfly has gone his own way, I have my students tell me as many details as they can remember. Then we look in Field Guides and on the Internet. There we will find photo for them to sketch from. Using this method even the youngest or least able student can take the time attend to the butterfly's details.
Tammy has a wonderful step-by-step method for painting butterflies here.