My old high school did Pygmalion a few years ago as a joint production with another one of the local high schools. For the first act, the kids from the other school were playing all the parts, and for the second act the cast consisted of kids from my school.
Either way, due to how similar My Fair Lady is to Pygmalion, I feel that Pygmalion might be a good place for you to start since your experience lies mostly in the musical department. All of the kids in the double cast played the parts well, so I don't think the material would be too difficult for your students, either.
"Was uns befreit, das muss stärker sein als wir es sind." -Tanz der Vampire