There have never been more resources in composition pedagogy. Here is a list of books on teaching composition that teachers have found helpful:
Composing Our Future: Preparing Music Educators to Teach Composition, ed. by Michele Kaschub and Janice Smith
The Mind’s Ear: Exercises for Improving the Musical Imagination for Performers, Composers, and Listeners, by Bruce Adolphe
Minds on Music: Composition for Creative and Critical Thinking, by Michele Kaschub and Janice Smith
Music Outside the Lines: Ideas for Composing in K-12 Music Classrooms, by Maud Hickey
Musicianship: Composing in Band and Orchestra, ed. by Clint Randles and David Stringham
Teaching Music Through Composition: A Curriculum Using Technology, by Barbara Freedman
Using Technology to Unlock Musical Creativity, by Scott Watson
Why and How to Teach Music Composition, ed. by Maud Hickey