Hi, welcome to HCM.
As a teenager, if you are spitting up phlegm too much then you need to really consider it seriously. if its been there for a long time, there can be possibilities of inflammation in the respiratory tract
or any malfuntioning. Being a passive smoker might also cause this. Sometimes excessive intake of dairy products can cause phlegm. Also avoid consuming lot of junk and fatty foods as they might increase your weight which in turn might put stress
on your respiratory system. The situation needs a treatment and hence would advice you to visit a doc at the earliest.