At our school, we generally teach this play to 9th graders; however, I think an advanced 8th-grade class could easily handle the text.
This is a great work for studying characterization and types of characters. Be sure to stop often and ask questions about each character and how the reader knows about them--what is directly told, what can they infer, etc. What ways do the characters grow or stay the same?
It would be helpful to give the students some background information about Helen Keller. I have also had students simulate handicaps with blindfolds and earplugs and let them try to communicate with each other in pairs or have one student lead the "handicapped" student to the water fountain.
You can talk about how handicapped students are treated in today's schools as opposed to those in Keller's day. What are some suggestions for dealing with different needs in education?
Enotes has FANTASTIC lesson plans and teachers' guides. Check out the link below for lessons for The Miracle Worker.