If you are currently working with early adolescent singers, there is a good chance that the textbooks you studied in college were written about how to teach high school age singers. You may never have had the opportunity to read a book specifically designed to speak to the needs of middle-level age singers.
Three books which will be very helpful to you are recommended here.
The first is The Cambiata Concept by Don L. Collins, Professor of Choral Music Education at the University of Central Arkansas. Dr. Collins provides a very concise, to the point, in-depth look at the early adolescent male and female changing voice, vocal technique, choral singing, voice classification, singing in unison, sight reading, rote teaching, the uncertain singer, and proper choral literature. This is the type book that will become your "handbook about adolescent singers" and will be left on your desk for immediate reference.
The second is the new 1999 edition of the choral music methods textbook entitled Teaching Choral Music , 2nd ed., also by Dr. Collins. It is one of the most comprehensive publications about how to teach choral music on the market today. It includes chapters on the European roots of choral music, choral music in America, developing a strategy for teaching choral music, characteristics of a master teacher, adolescent boys' and girls' changing voices, understanding the adolescent, proper vocal technique for adolescent voices, getting the most out of rehearsal, classes with and without emphasis on performance, the unique sound of adolescents singing together, the choral environment, and much more.
The third is Teaching Junior High School Music by Irvin Cooper,former Professor of Music at Florida State University and Karl Kuersteiner, former Dean of the School of Music at Florida State University. This book is the original authoritative writing about the Cambiata Concept. Originally written in 1965, it remains one of the most quoted texts on dealing with adolescent voices found in libraries of music educators today. A second edition was released in 1970.
The books deal with the concept of general music, a singing program, voice classification, a music reading program, an ear-training program, choral techniques, fundamentals of music, a listening program, the story of music (history), and responsibilities of the vocal music teacher in middle and junior high school.
The Cambiata Concept and Teaching Junior High School Music are published by Cambiata Press, P.O. Box 1151, Conway Arkansas 72032. Teaching Choral Music, 2nd ed., is published by Prentice-Hall, Upper Saddle River, NJ. All three may be obtained from J.W. Pepper at 800-345-6296 or your favorite music dealer.