What could cause frequent bloody nose in teen?

It is due to the fragility of the vessels, called Kiesselbach plexus, which supply blood to the anterior inferior (lower front) quadrant of the nasal septum. This nosebleed is quite common, exacerbated with dry air. If bleeding is too frequent and bothersome, that plexus can be cauterized by an ENT doctor. Apply bland ointment like K-jelly to that location may be helpful as a prevention.

Trauma. Often minor. The tiny capillaries in the nasal membranes are fragile. Allergies/colds/etc. trigger wiping or rubbing, often breaking these vessels, wakening to blood stains in a pillow is common to those afflicted.

Th There are multiple possible conditions, including local inflammation and rupture of small vessels. Other conditions such as a bleeding disorder if the bleeding is more than minutes with pressure applied should be considered. A healthy diet including sources of Vitamin K and C ar recommended.