Migraine with aura: This condition is common in adolescents and can cause anisocoria (unequal pupil sizes) along with headache. The aura symptoms can include visual disturbances, which might be accompanied by pupillary changes.

Cluster headache: Although less common in adolescents, cluster headaches can cause anisocoria on the same side as the headache, along with severe pain.
Horner's syndrome: This can be a result of various conditions and causes anisocoria, ptosis (drooping eyelid), and decreased sweating on one side of the face. It might be accompanied by headache.
Conjunctivitis or uveitis: Inflammation of the eye can cause anisocoria due to irritation or pain leading to pupil constriction on the affected side, and it might be accompanied by headache.

Subarachnoid hemorrhage: Although rare in adolescents, a sudden, severe headache (often described as "the worst headache of my life") with anisocoria could indicate a life-threatening condition like subarachnoid hemorrhage.
Brain tumor: A tumor pressing on the brainstem or cranial nerves can cause anisocoria and headache. Early detection is crucial for treatment.
Cavernous sinus thrombosis: This is a rare but serious condition that can cause anisocoria, headache, and other neurological symptoms due to a blood clot in the cavernous sinus.

Adie's pupil (Tonic pupil): A rare condition causing anisocoria due to abnormal pupil function, often accompanied by decreased deep tendon reflexes.
Syphilis: In its tertiary stage, syphilis can cause a range of neurological symptoms, including anisocoria and headache, although this would be extremely rare in a 13-year-old without specific risk factors.
Multiple sclerosis: Although primarily causing neurological symptoms other than anisocoria and headache, multiple sclerosis can rarely present with these symptoms, especially in the context of optic neuritis.