This strikes me as partly a training issue if a few have this problem - do they dynamically warm up, do they perform a cool down and flexibilty work. MAin thing i find at football academies is the players get tight then do a lot of training and get sore.
You should definitely check all main muscle groups around the pelvis - quads/hip flexors/ hamstrings. Also checking lower limb biomechanical control - eg small knee bends. And spinal mobility in thoracic spine and lumbar. I also find there general stability isn't always brilliant.
Also at that age posture - school bag over one shoulder, long days slouched etc may influence there symptoms.