Stress hormones. The same hormones that cause lightheadedness at any age, the stress hormones/adrenaline. Adolescence is a time when we tend to think in extremes and are prone to over react to minor events or have social anxiety. A psychologist trained in cbt can teach you to think in a healthy way and control your anxiety and the stress hormones that come with anxiety.
POTS. The most common cause of constant lightheadedness during adolescence is the developmental form of a condition called postural orthostatic tachycardia syndrome. This occurs during periods off rapid growth when the autonomic nervous system has trouble adjusting to compensate for growth. It goes away in time if it is the benign developmental type. In the meantime you can feel better by drinking more water (about 2 liters a day more than you do now) and adding extra salt to your food and your diet. If this does not help, see your doctor.