Viral Meningitis: This is a common cause of fever and headache in adolescents, often presenting with symptoms such as fever, headache, stiff neck, and sometimes vomiting. The justification for this being the most likely diagnosis is the age of the patient and the combination of symptoms presented.

Influenza: Given the symptoms of fever and headache, influenza is a plausible diagnosis, especially during flu season. It's a common illness that can present with these symptoms, among others like cough and body aches.
Sinusitis: This condition can cause fever and headache, particularly if the headache is localized to the facial area. It's a common condition in adolescents and can be triggered by viral or bacterial infections.
Mononucleosis (Mono): Caused by the Epstein-Barr virus, mono can present with fever, headache, and sometimes a sore throat or lymphadenopathy. It's more common in adolescents and young adults.

Bacterial Meningitis: Although less common than viral meningitis, bacterial meningitis is a medical emergency that requires prompt treatment. It presents similarly to viral meningitis but can have a more severe course and higher risk of complications or death if not treated immediately.
Encephalitis: An inflammation of the brain tissue, usually caused by viral infections. It's less common but can be life-threatening, presenting with fever, headache, confusion, and sometimes seizures.
Subarachnoid Hemorrhage: A bleeding into the space surrounding the brain, which can cause a sudden, severe headache (often described as "the worst headache" ever experienced) and fever. It's rare but critical to diagnose and treat promptly.

Brain Abscess: A rare condition where an abscess forms in the brain, usually as a result of a bacterial or fungal infection. It can present with fever, headache, and neurological deficits, depending on the location of the abscess.
Lymphoma or Leukemia: Although rare, these cancers can present with systemic symptoms such as fever and headache, among others like weight loss or lymphadenopathy.
Autoimmune Encephalitis: A group of conditions that occur when the body's immune system mistakenly attacks healthy brain cells, leading to inflammation of the brain. It's rare and can present with a variety of neurological and psychiatric symptoms, including fever and headache.