Asana Alphabet recommends yoga mats in bulk for schools--they are more economical and you can cut them to the actual size that you want (click on image to purchase). 

Asana Alphabet recommended reading--click on a title for info/purchase:

Yogi and Yogette Learn the Asana Alphabet

Smart Moves by Carla Hannaford, PhD

Yoga Nidra by Swami Satyananda Saraswati

Theater Games for the Classroom by Viola Spolin

Yoga for the Special Child, Sonia Sumar

Kids' Mandalas

Meditation as Medicine (Great for working with teens)

Yoga Journal Pose Encyclopedia (Esp. for teachers newer to yoga)

Yoga Journals (for middle and high school students).

Or you may want to opt for a yoga blanket that can be folded into the size you want to use