There are many different causes of fatigue and feelings of increased sleepiness. It is true that depression and other systemic conditions can result in sleepiness, but there are often more benign conditions as the etiology to fatigue. You should speak with your primary care doctor
regarding your symptoms and need for further testing or workup.
There is an entity known as chronic fatigue syndrome which is fairly rare and is characterized by chronic fatigue lasting at least 6 months which results in impairment of physical and mental functioning. Other conditions that can cause fatigue include fibromyalgia, anxiety
disorders, depression, hypothyroidism, Lyme disease, Epstein Barr Virus, and other infectious diseases.
However, these conditions are rare causes of fatigue in adolescents. It is great that you are exercising and eating well because nutrition and exercise are important components to a healthy lifestyle and ability to sleep. Most commonly, high school students have poor sleep hygiene. You should try to go to bed at the same time every night, including weekends, and try to have at least 8 hours of sleep a night.
It is important to pay attention to any sleeping disturbances - such as difficulty going to sleep, noise at night, need to urinate during the night, and difficulty staying asleep. You should discuss these details with your primary physician to see if further workup or management is required.