A recent study by the Department of Orthopaedic Surgery, University of California, revealed backpacks increased the load on the lumbar spine significantly and student subjects reported increases in back pain.
Another study on German primary school children compared rolling luggage to carrying schoolbooks in a backpack. It turned out that while both methods place strain on the spine, pulling a trolley increases spinal rotation. The report concluded that "schoolchildren should use backpacks rather than trolleys when the weight is within recommended limits."
The key message is to keep the weight in the backpack as low as possible in order to reduce stress and strain on the developing spine.