Osgood-Schlatter Disease: This condition is a common cause of knee pain in adolescents, particularly during growth spurts. It is characterized by inflammation of the patellar tendon at its insertion on the tibial tubercle, often due to overuse or repetitive stress from activities such as running or jumping.

Patellofemoral Pain Syndrome: This condition is also common in adolescents and is characterized by pain around or behind the patella, often related to activities that put stress on the knee, such as climbing stairs, jumping, or squatting. It can be due to various factors including overuse, poor tracking of the patella, or muscle imbalances.
Medial Collateral Ligament (MCL) Sprain or Strain: Although the question specifies no known injury, minor incidents might not always be reported or recognized. An MCL sprain could cause medial knee pain.
Meniscal Tear: Though less common in this age group without a significant injury, a meniscal tear could cause knee pain, especially if there's a history of twisting injuries or sports participation.

Septic Arthritis: This is a medical emergency that requires prompt diagnosis and treatment. It can present with sudden onset of knee pain, swelling, redness, and warmth, often accompanied by systemic symptoms such as fever.
Osteosarcoma or Other Bone Tumors: Although rare, bone tumors can cause knee pain and must be considered, especially if the pain is persistent, worsening, or accompanied by other systemic symptoms like weight loss or fatigue.
Slipped Capital Femoral Epiphysis (SCFE): This condition, more common in obese adolescents, involves the displacement of the femoral head from the femoral neck through the physeal plate. It can cause hip pain that may be referred to the knee.

Juvenile Rheumatoid Arthritis (JRA): Now more commonly referred to as Juvenile Idiopathic Arthritis (JIA), this condition can cause knee pain and swelling, among other symptoms, and is diagnosed after other causes are ruled out.
Kohler Disease: A rare condition causing avascular necrosis of the navicular bone in the foot but can sometimes refer pain to the knee due to altered biomechanics.
Chronic Recurrent Multifocal Osteomyelitis (CRMO): A rare condition that can cause bone pain, including in the knee, due to recurrent episodes of osteomyelitis without an infectious cause.