Scheuermann's Kyphosis: A common condition in adolescents characterized by kyphosis of the spine, which can cause back pain that is worse in the morning and improves with activity. The chronic nature of the pain and the age of the patient make this a likely diagnosis.

Mechanical Low Back Pain: This is a broad category that includes pain due to muscle strain, poor posture, or other mechanical issues. It's common in adolescents and can present with morning stiffness and improvement with activity.
Scoliosis: While often asymptomatic, scoliosis can cause back pain, especially if the curvature is significant. The pain might be worse in the morning due to prolonged periods of rest.
Musculoskeletal Strain: Adolescents can experience back pain due to overuse or strain from sports or other activities, which might fit the described pattern of pain.

Spinal Tumors: Although rare, tumors such as osteoid osteoma, osteoblastoma, or spinal cord tumors can cause back pain that is worse at night or in the morning. Missing these diagnoses could have severe consequences.
Infectious Diseases (e.g., Discitis, Osteomyelitis): Infections of the spine can cause severe back pain, often worse at night, and can lead to serious complications if not treated promptly.
Inflammatory Back Pain (e.g., Ankylosing Spondylitis): This condition can cause chronic back pain and stiffness, particularly in the morning, and is associated with significant long-term disability if not properly managed.

Ehlers-Danlos Syndrome: A genetic disorder that can affect the connective tissue and lead to joint hypermobility and musculoskeletal pain, including back pain.
Marfan Syndrome: Another genetic disorder that can affect the connective tissue, leading to musculoskeletal symptoms including back pain due to spinal abnormalities.
Osteogenesis Imperfecta: A condition characterized by brittle bones, which can lead to back pain due to vertebral fractures or deformities.