Flat pillow might be okay with a student. The pillow to prop it head high or too low will press the neck bones, neck muscle causes tension and pain, especially when lying upside down or it's.
Flat pillow may not be okay with a collar. Bolster pillow is too high or low, then head to the neck causes the neck muscles, stiffness and pain. Especially prone or side.
Pillow may not okay with the neck. A pillow that support and the head is too low or too high. To press the collar bone, causing the neck muscles and pain. The sleep upside down or on its side.